ucdavis, ecs150 fall 2007 11/13/2007ecs150, fall 20071 operating system ecs150 fall 2007 : operating...
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11/13/2007 ecs150, Fall 2007 1
UCDavis, ecs150Fall 2007
ecs150 Fall 2007:Operating SystemOperating System#5: File Systems(chapters: 6.4~6.7, 8)
Dr. S. Felix Wu
Computer Science Department
University of California, Davishttp://www.cs.ucdavis.edu/~wu/
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UCDavis, ecs150Fall 2007
File System AbstractionFile System Abstraction
Files Directories
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UCDavis, ecs150Fall 2007
System-call interfaceActive file entries
VNODE Layer or VFS
Local naming (UFS)
FFS
Buffer cache
Block or character device driver
Hardware
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UCDavis, ecs150Fall 2007
dirp = opendir(const char *filename);struct dirent *direntp = readdir(dirp);
struct dirent {ino_t d_ino;char d_name[NAME_MAX+1];
};
directory
direntinode
file_name
file
file
file
direntinode
file_name
direntinode
file_name
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UCDavis, ecs150Fall 2007
Local versus RemoteLocal versus Remote
System Call Interface V-node Local versus remote
– NFS or i-node– Stackable File System
Hard-disk blocks
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File-System StructureFile-System Structure File structure
– Logical storage unit– Collection of related information
File system resides on secondary storage (disks).
File system organized into layers. File control block – storage structure
consisting of information about a file.
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UCDavis, ecs150Fall 2007 File File Disk Disk
separate the disk into blocks separate the file into blocks as well paging from file to disk
blocks: 4 - 7- 2- 10- 12
How to represent the file??How to link these 5 pages together??
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Bit torrent piecesBit torrent pieces
1 big file (X Gigabytes) with a number of pieces (5%) already in (and sharing with others).
How much disk space do we need at this moment?
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UCDavis, ecs150Fall 2007 Hard DiskHard Disk
Track, Sector, Head– Track + Heads Cylinder
Performance– seek time– rotation time– transfer time
LBA– Linear Block Addressing
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UCDavis, ecs150Fall 2007 File File Disk blocks Disk blocks
fileblock
0
4
fileblock
1
7
fileblock
2
2
fileblock
3
10
0file
block4
12
What are the disadvantages?1. disk access can be slow for “random access”.2. How big is each block? 64 bytes? 68 bytes?
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UCDavis, ecs150Fall 2007
Kernel Hacking SessionKernel Hacking Session
This Friday from 7:30 p.m. until midnight.. 3083 Kemper
– Bring your laptop– And bring your mug…
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UCDavis, ecs150Fall 2007 A File SystemA File System
partition partition partition
i-list directory and data blockssb
i-node i-node ……. i-node
d
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UCDavis, ecs150Fall 2007
One Logical File One Logical File Physical Disk Blocks Physical Disk Blocks
efficient representation & access
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UCDavis, ecs150Fall 2007 An i-nodeAn i-node
Typical:each block 8K or 16K bytes
??? entries inone disk block
A file
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UCDavis, ecs150Fall 2007
inode (index node) structureinode (index node) structure meta-data of the file.
– di_mode 02– di_nlinks 02– di_uid 02– di_gid 02– di_size 04– di_addr 39– di_gen 01– di_atime 04– di_mtime 04– di_ctime 04
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UCDavis, ecs150Fall 2007
System-call interfaceActive file entries
VNODE Layer or VFS
Local naming (UFS)
FFS
Buffer cache
Block or character device driver
Hardware
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UCDavis, ecs150Fall 2007 A File SystemA File System
partition partition partition
i-list directory and data blockssb
i-node i-node ……. i-node
d
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UCDavis, ecs150Fall 2007
125 struct ufs2_dinode {126 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */127 int16_t di_nlink; /* 2: File link count. */128 u_int32_t di_uid; /* 4: File owner. */ 129 u_int32_t di_gid; /* 8: File group. */ 130 u_int32_t di_blksize; /* 12: Inode blocksize. */ 131 u_int64_t di_size; /* 16: File byte count. */ 132 u_int64_t di_blocks; /* 24: Bytes actually held. */ 133 ufs_time_t di_atime; /* 32: Last access time. */ 134 ufs_time_t di_mtime; /* 40: Last modified time. */ 135 ufs_time_t di_ctime; /* 48: Last inode change time. */ 136 ufs_time_t di_birthtime; /* 56: Inode creation time. */ 137 int32_t di_mtimensec; /* 64: Last modified time. */ 138 int32_t di_atimensec; /* 68: Last access time. */ 139 int32_t di_ctimensec; /* 72: Last inode change time. */ 140 int32_t di_birthnsec; /* 76: Inode creation time. */ 141 int32_t di_gen; /* 80: Generation number. */ 142 u_int32_t di_kernflags; /* 84: Kernel flags. */ 143 u_int32_t di_flags; /* 88: Status flags (chflags). */ 144 int32_t di_extsize; /* 92: External attributes block. */ 145 ufs2_daddr_t di_extb[NXADDR];/* 96: External attributes block. */ 146 ufs2_daddr_t di_db[NDADDR]; /* 112: Direct disk blocks. */ 147 ufs2_daddr_t di_ib[NIADDR]; /* 208: Indirect disk blocks. */ 148 int64_t di_spare[3]; /* 232: Reserved; currently unused */ 149 };
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UCDavis, ecs150Fall 2007166 struct ufs1_dinode {
167 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */ 168 int16_t di_nlink; /* 2: File link count. */ 169 union { 170 u_int16_t oldids[2]; /* 4: Ffs: old user and group ids. */ 171 } di_u; 172 u_int64_t di_size; /* 8: File byte count. */ 173 int32_t di_atime; /* 16: Last access time. */ 174 int32_t di_atimensec; /* 20: Last access time. */ 175 int32_t di_mtime; /* 24: Last modified time. */ 176 int32_t di_mtimensec; /* 28: Last modified time. */ 177 int32_t di_ctime; /* 32: Last inode change time. */ 178 int32_t di_ctimensec; /* 36: Last inode change time. */ 179 ufs1_daddr_t di_db[NDADDR]; /* 40: Direct disk blocks. */ 180 ufs1_daddr_t di_ib[NIADDR]; /* 88: Indirect disk blocks. */ 181 u_int32_t di_flags; /* 100: Status flags (chflags). */ 182 int32_t di_blocks; /* 104: Blocks actually held. */ 183 int32_t di_gen; /* 108: Generation number. */ 184 u_int32_t di_uid; /* 112: File owner. */ 185 u_int32_t di_gid; /* 116: File group. */ 186 int32_t di_spare[2]; /* 120: Reserved; currently unused */ 187 };
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Bittorrent piecesBittorrent pieces
File size: 10 GBPieces downloaded: 512 MBHow much disk space do we need?
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#include <stdio.h>#include <stdlib.h>
intmain(void){ FILE *f1 = fopen("./sss.txt", "w"); int i;
for (i = 0; i < 1000; i++) { fseek(f1, rand(), SEEK_SET); fprintf(f1, "%d%d%d%d", rand(), rand(), rand(), rand()); if (i % 100 == 0) sleep(1); } fflush(f1);}
# ./t# ls –l ./sss.txt
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UCDavis, ecs150Fall 2007 An i-nodeAn i-node
Typical:each block 1K
??? entries inone disk block
A file
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UCDavis, ecs150Fall 2007
i-nodei-node
How many disk blocks can a FS have? How many levels of i-node indirection will be
necessary to store a file of 2G bytes? (I.e., 0, 1, 2 or 3) What is the largest possible file size in i-node? What is the size of the i-node itself for a file of 10GB
with only 512 MB downloaded?
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AnswerAnswer How many disk blocks can a FS have?
– 264 or 232: Pointer (to blocks) size is 8/4 bytes. How many levels of i-node indirection will be
necessary to store a file of 2G (231) bytes? (I.e., 0, 1, 2 or 3)– 12*210 + 28 * 210 + 28 *28 *2 10 + 28 * 28 *28 *2 10 >? 231
What is the largest possible file size in i-node?– 12*210 + 28 * 210 + 28 *28 *2 10 + 28 * 28 *28 *2 10
– 264 –1– 232 * 210
You need to consider three issues and find the minimum!
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Answer: Lower BoundAnswer: Lower Bound
How many pointers?– 512MB divided by the block size (1K)– 512K pointers times 8 (4) bytes = 4 (2) MB
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Bittorrent piecesBittorrent pieces
File size: 10 GBPieces downloaded: 512 MBHow much disk space do we need?
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Answer: Upper BoundAnswer: Upper Bound
In the worst case, EVERY indirection block has at least one entry!
How many indirection blocks?– Single: 1 block– Double: 1 + 28
– Tripple: 1 + 28 + 216
Total ~ 216 blocks times 1K = 64 MB– 214 times 1K = 16MB (ufs2 inode)
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Answer (4)Answer (4)
2 MB ~ 64 MB ufs1 4 MB ~ 16 MB ufs2
Answer: sss.txt ~17 MB– ~16 MB (inode indirection blocks)– 1000 writes times 1K ~ 1MB
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UCDavis, ecs150Fall 2007 An i-nodeAn i-node
Typical:each block 1K
??? entries inone disk block
A file
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UCDavis, ecs150Fall 2007 A File SystemA File System
partition partition partition
i-list directory and data blockssb
i-node i-node ……. i-node
d
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UCDavis, ecs150Fall 2007
FFS and UFSFFS and UFS
/usr/src/sys/ufs/ffs/*– Higher-level: directory structure– Soft updates & Snapshot
/usr/src/sys/ufs/ufs/*– Lower-level: buffer, i-node
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# of i-nodes# of i-nodes
UFS1: pre-allocation– 3% of HD, about < 25% used.
UFS2: dynamic allocation– Still limited # of i-nods
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di_size vs. di_blocksdi_size vs. di_blocks
???
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One Logical File One Logical File Physical Disk Blocks Physical Disk Blocks
efficient representation & access
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di_size vs. di_blocksdi_size vs. di_blocks
Logical Physical
fstat du
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Extended Attributes in UFS2Extended Attributes in UFS2 Attributes associated with the File
– di_extb[2]; – two blocks, but indirection if needed.
Format– Length 4– Name Space 1– Content Pad Length 1– Name Length 1– Name mod 8– Content variable
Applications: ACL, Data Labelling
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Some thoughts….Some thoughts…. What can you do with “extended attributes”? How to design/implement?
– Should/can we do it “Stackable File Systems”?– Otherwise, the program to manipulate the EA’s
will have to be very UFS2-dependent or FiST with an UFS2 optimization option.
Are there any counter examples?– security and performance considerations.
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UCDavis, ecs150Fall 2007 A File SystemA File System
partition partition partition
i-list directory and data blockssb
i-node i-node ……. i-node
d
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UCDavis, ecs150Fall 2007 struct dirent {
ino_t d_ino;char d_name[NAME_MAX+1];
};
struct stat {…short nlinks;
…};
directory
direntinode
file_name
file
file
file
direntinode
file_name
direntinode
file_name
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drwxr-xr-xApr 1 2004
root wheel
drwxr-xr-xApr 1 2004
root wheel
rwxr-xr-xApr 15 2004
root wheel
rw-rw-r--Jan 19 2004
kirk staff
drwxr-xr-xApr 1 2004
root wheel
rwxr-xr-xApr 15 2004
bin bin
2
3
4
5
6
7
8
9
. 2
.. 2usr 4
vmunix 5
. 4
.. 2bin 7foo 6
text data
Hello World!
. 7
.. 4ex 9
groff 10vi 9
text data
directory/
directory/usr
directory/usr/bin
file/vmunix
file/usr/foo
file/usr/bin/vi
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What is the difference?What is the difference?
ln –s /usr/src/sys/sys/proc.h ppp.h ln /usr/src/sys/sys/proc.h ppp.h
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Hard versus SymbolicHard versus Symbolic
ln –s /usr/src/sys/sys/proc.h ppp.h– Link to anything, any mounted partitions– Delete a Symbolic link?
ln /usr/src/sys/sys/proc.h ppp.h– Link only to “file” (not directory)– Link only within the same partition -- why?– Delete a Hard Link?
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125 struct ufs2_dinode {126 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */127 int16_t di_nlink; /* 2: File link count. */128 u_int32_t di_uid; /* 4: File owner. */ 129 u_int32_t di_gid; /* 8: File group. */ 130 u_int32_t di_blksize; /* 12: Inode blocksize. */ 131 u_int64_t di_size; /* 16: File byte count. */ 132 u_int64_t di_blocks; /* 24: Bytes actually held. */ 133 ufs_time_t di_atime; /* 32: Last access time. */ 134 ufs_time_t di_mtime; /* 40: Last modified time. */ 135 ufs_time_t di_ctime; /* 48: Last inode change time. */ 136 ufs_time_t di_birthtime; /* 56: Inode creation time. */ 137 int32_t di_mtimensec; /* 64: Last modified time. */ 138 int32_t di_atimensec; /* 68: Last access time. */ 139 int32_t di_ctimensec; /* 72: Last inode change time. */ 140 int32_t di_birthnsec; /* 76: Inode creation time. */ 141 int32_t di_gen; /* 80: Generation number. */ 142 u_int32_t di_kernflags; /* 84: Kernel flags. */ 143 u_int32_t di_flags; /* 88: Status flags (chflags). */ 144 int32_t di_extsize; /* 92: External attributes block. */ 145 ufs2_daddr_t di_extb[NXADDR];/* 96: External attributes block. */ 146 ufs2_daddr_t di_db[NDADDR]; /* 112: Direct disk blocks. */ 147 ufs2_daddr_t di_ib[NIADDR]; /* 208: Indirect disk blocks. */ 148 int64_t di_spare[3]; /* 232: Reserved; currently unused */ 149 };
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UCDavis, ecs150Fall 2007 struct dirent {
ino_t d_ino;char d_name[NAME_MAX+1];
};
struct stat {…short nlinks;
…};
directory
direntinode
file_name
file
file
file
direntinode
file_name
direntinode
file_name
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File System Buffer CacheFile System Buffer Cacheapplication: read/write files
OS: translate file to disk blocks
...buffer cache ...maintains
controls disk accesses: read/write blocks
hardware:
Any problems?
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File System ConsistencyFile System Consistency
To maintain file system consistency the ordering of updates from buffer cache to disk is critical
Example:– if the directory block is written back before the
i-node and the system crashes, the directory structure will be inconsistent
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File System ConsistencyFile System Consistency File system almost always use a buffer/disk cache for
performance reasons This problem is critical especially for the blocks that
contain control information: i-node, free-list, directory blocks
Two copies of a disk block (buffer cache, disk) consistency problem if the system crashes before all the modified blocks are written back to disk
Write back critical blocks from the buffer cache to disk immediately
Data blocks are also written back periodically: sync
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Two StrategiesTwo Strategies Prevention
– Use un-buffered I/O when writing i-nodes or pointer blocks
– Use buffered I/O for other writes and force sync every 30 seconds
Detect and Fix– Detect the inconsistency
– Fix them according to the “rules”
– Fsck (File System Checker)
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File System IntegrityFile System Integrity Block consistency:
– Block-in-use table
– Free-list table
File consistency:– how many directories pointing to that i-node?
– nlink?
– three cases: D == L, L > D, D > L What to do with the latter two cases?
0 1 1 1 0 0 0 1 0 0 0 2
1 0 0 0 1 1 1 0 1 0 2 0
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UCDavis, ecs150Fall 2007 File System IntegrityFile System Integrity
File system states(a) consistent(b) missing block(c) duplicate block in free list(d) duplicate data block
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Metadata OperationsMetadata Operations
Metadata operations modify the structure of the file system– Creating, deleting, or renaming
files, directories, or special files– Directory & I-node
Data must be written to disk in such a way that the file system can be recovered to a consistent state after a system crash
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Metadata IntegrityMetadata Integrity
FFS uses synchronous writes to guarantee the integrity of metadata– Any operation modifying multiple pieces of
metadata will write its data to disk in a specific order
– These writes will be blocking Guarantees integrity and durability of
metadata updates
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Deleting a file (I)Deleting a file (I)
abc
def
ghi
i-node-1
i-node-2
i-node-3
Assume we want to delete file “def”
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Deleting a file (II)Deleting a file (II)
abc
def
ghi
i-node-1
i-node-3
Cannot delete i-node before directory entry “def”
?
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Deleting a file (III)Deleting a file (III)
Correct sequence is1. Write to disk directory block containing deleted
directory entry “def”
2. Write to disk i-node block containing deleted i-node
Leaves the file system in a consistent state
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Creating a file (I)Creating a file (I)
abc
ghi
i-node-1
i-node-3
Assume we want to create new file “tuv”
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Creating a file (II)Creating a file (II)
abc
ghi
tuv
i-node-1
i-node-3
Cannot write directory entry “tuv” before i-node
?
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Creating a file (III)Creating a file (III)
Correct sequence is1. Write to disk i-node block containing new i-node
2. Write to disk directory block containing new directory entry
Leaves the file system in a consistent state
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Synchronous UpdatesSynchronous Updates
Used by FFS to guarantee consistency of metadata:– All metadata updates are done through blocking
writes
Increases the cost of metadata updates Can significantly impact the performance
of whole file system
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SOFT UPDATESSOFT UPDATES
Use delayed writes (write back) Maintain dependency information about
cached pieces of metadata:This i-node must be updated before/after this directory entry
Guarantee that metadata blocks are written to disk in the required order
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3 Soft Update Rules3 Soft Update Rules
Never point to a structure before it has been initialized.
Never reuse a resource before nullifying all previous pointers to it.
Never reset the old pointer to a live resource before the new pointer has been set.
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Problem #1 with S.U.Problem #1 with S.U.
Synchronous writes guaranteed that metadata operations were durable once the system call returned
Soft Updates guarantee that file system will recover into a consistent state but not necessarily the most recent one– Some updates could be lost
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We want to delete file “foo” and create new file “bar”
i-node-2 foo
NEW bar
NEW i-node-3
Block A Block B
What are the dependency relationship?
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We want to delete file “foo” and create new file “bar”
i-node-2 foo
NEW bar
NEW i-node-3
Block A Block B
Circular DependencyX-2nd Y-1st
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Problem #2 with S.U.Problem #2 with S.U.
Cyclical dependencies:– Same directory block contains entries to be
created and entries to be deleted– These entries point to i-nodes in the same block
Brainstorming:– How to resolve this issue in S.U.?
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FS: buffer or disk??FS: buffer or disk??
They appear in both and we try to synchronize them..
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DiskDisk
i-node-2 foo
Block A-Dir Block B-i-Node
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BufferBuffer
NEW bar
NEW i-node-3
Block A-Dir Block B-i-Node
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Synchronize??Synchronize??
i-node-2 foo
NEW bar
NEW i-node-3
Block A Block B
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How to update?? i-node first or director block first?
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Solution in S.U.Solution in S.U.
Roll back metadata in one of the blocks to an earlier, safe state
(Safe state does not contain new directory entry)
def
Block A’
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Write first block with metadata that were rolled back (block A’ of example)
Write blocks that can be written after first block has been written (block B of example)
Roll forward block that was rolled back Write that block Breaks the cyclical dependency but must now
write twice block A
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Before any Write Operation
After any Write Operation
SU Dependency Checking(roll back if necessary)
SU Dependency Processing(task list updating)(roll forward if necessary)
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two most popular approaches for improving the performance of metadata operations and recovery:– Journaling – Soft Updates
Journaling systems record metadata operations on an auxiliary log
Soft Updates uses ordered writes
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UCDavis, ecs150Fall 2007 JOURNALINGJOURNALING
Journaling systems maintain an auxiliary log that records all meta-data operations
Write-ahead logging ensures that the log is written to disk before any blocks containing data modified by the corresponding operations.– After a crash, can replay the log to bring the file
system to a consistent state
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JOURNALINGJOURNALING
Log writes are performed in addition to the regular writes
Journaling systems incur log write overhead but– Log writes can be performed efficiently
because they are sequential (block operation consideration)
– Metadata blocks do not need to be written back after each update
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JOURNALINGJOURNALING
Journaling systems can provide– same durability semantics as FFS if log is
forced to disk after each meta-data operation– the laxer semantics of Soft Updates if log
writes are buffered until entire buffers are full
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Soft Updates vs. JournalingSoft Updates vs. Journaling
Advantages disadvantages
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With Soft Updates??With Soft Updates??
CPU
Do we still need “FSCK”? at boot time?
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Recover the Missing ResourcesRecover the Missing Resources
In the background, in an active FS…– We don’t want to wait for the lengthy FSCK
process to complete…
A related issue:– the virus scanning process– what happens if we get a new virus signature?
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Snapshot of the FSSnapshot of the FS
backup and restore dump reliably an active File System
– what will we do today to dump our 40GB FS “consistent” snapshots? (in the midnight…)
“background FSCK checks”
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What is a snapshot?What is a snapshot?(I mean “conceptually”.)(I mean “conceptually”.)
Freeze all activities related to the FS. Copy everything to “some space”. Resume the activities.
How do we efficiently implement this concept such that the activities will only be blocked for about 0.25 seconds, and we don’t have to buy a really big hard drive?
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UCDavis, ecs150Fall 2007 Snapshot: a fileSnapshot: a file
Logical sizeVersus physical size
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ExampleExample
# mkdir /backups/usr/noon# mount –u –o snapshot /usr/snap.noon /usr# mdconfig –a –t vnode –u 0 –f /usr/snap.noon# mount –r /dev/md0 /backups/usr/noon
/* do whatever you want to test it */
# umount /backups/usr/noon# mdconfig –d –u 0# rm –f /usr/snap.noon
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#include <stdio.h>#include <stdlib.h>
intmain(void){ FILE *f1 = fopen("./sss.txt", "w"); int i;
for (i = 0; i < 1000; i++) { fseek(f1, rand(), SEEK_SET); fprintf(f1, "%d%d%d%d", rand(), rand(), rand(), rand()); if (i % 100 == 0) sleep(1); } fflush(f1);}
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ExampleExample
# mkdir /backups/usr/noon# mount –u –o snapshot /usr/snap.noon /usr# mdconfig –a –t vnode –u 0 –f /usr/snap.noon# mount –r /dev/md0 /backups/usr/noon
/* do whatever you want to test it */
# umount /backups/usr/noon# mdconfig –d –u 0# rm –f /usr/snap.noon
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ExampleExample
# mkdir /backups/usr/noon# mount –u –o snapshot /usr/snap.noon /usr# mdconfig –a –t vnode –u 0 –f /usr/snap.noon# mount –r /dev/md0 /backups/usr/noon
/* do whatever you want to test it */
# umount /backups/usr/noon# mdconfig –d –u 0# rm –f /usr/snap.noon
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UCDavis, ecs150Fall 2007 A File SystemA File System
??? entries inone disk block
A file
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UCDavis, ecs150Fall 2007 A Snapshot i-nodeA Snapshot i-node
??? entries inone disk block
A file
Not used orNot yet copy
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UCDavis, ecs150Fall 2007 Copy-on-writeCopy-on-write
??? entries inone disk block
A file
Not used orNot yet copy
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UCDavis, ecs150Fall 2007 Copy-on-writeCopy-on-write
??? entries inone disk block
A file
Not used orNot yet copy
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Multiple SnapshotsMultiple Snapshots
about 20 snapshots Interactions/sharing among snapshots
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Snapshot of the FSSnapshot of the FS
backup and restore dump reliably an active File System
– what will we do today to dump our 40GB FS “consistent” snapshots? (in the midnight…)
“background FSCK checks”
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VFS: the FS SwitchVFS: the FS Switch
syscall layer (file, uio, etc.)
user space
Virtual File System (VFS)networkprotocol
stack(TCP/IP) NFS FFS LFS etc.*FS etc.
device drivers
Sun Microsystems introduced the virtual file system interface in 1985 to accommodate diverse filesystem types cleanly.
VFS allows diverse specific file systems to coexist in a file tree, isolating all FS-dependencies in pluggable filesystem modules.
VFS was an internal kernel restructuringwith no effect on the syscall interface.
Incorporates object-oriented concepts:a generic procedural interface withmultiple implementations.
Based on abstract objects with dynamicmethod binding by type...in C.Other abstract interfaces in the kernel: device drivers,
file objects, executable files, memory objects.
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vnodevnode In the VFS framework, every file or directory in active use
is represented by a vnode object in kernel memory.
syscall layer
NFS UFS
free vnodes
Each vnode has a standardfile attributes struct.
Vnode operations aremacros that vector tofilesystem-specificprocedures.
Generic vnode points atfilesystem-specific struct(e.g., inode, rnode), seenonly by the filesystem. Each specific file system
maintains a cache of its resident vnodes.
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vnode Operations and vnode Operations and AttributesAttributes
directories onlyvop_lookup (OUT vpp, name)vop_create (OUT vpp, name, vattr)vop_remove (vp, name)vop_link (vp, name)vop_rename (vp, name, tdvp, tvp, name)vop_mkdir (OUT vpp, name, vattr)vop_rmdir (vp, name)vop_symlink (OUT vpp, name, vattr, contents)vop_readdir (uio, cookie)vop_readlink (uio)
files onlyvop_getpages (page**, count, offset)vop_putpages (page**, count, sync, offset)vop_fsync ()
vnode attributes (vattr)type (VREG, VDIR, VLNK, etc.)mode (9+ bits of permissions)nlink (hard link count)owner user IDowner group IDfilesystem IDunique file IDfile size (bytes and blocks)access timemodify timegeneration number
generic operationsvop_getattr (vattr)vop_setattr (vattr)vhold()vholdrele()
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Network File System (NFS)Network File System (NFS)
syscall layer
UFS
NFSserver
VFS
VFS
NFSclient
UFS
syscall layer
client
user programs
network
server
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vnode Cachevnode CacheHASH(fsid, fileid)
VFS free list headActive vnodes are reference- counted by the structures that hold pointers to them.
- system open file table
- process current directory
- file system mount points
- etc.
Each specific file system maintains its own hash of vnodes (BSD).
- specific FS handles initialization
- free list is maintained by VFSvget(vp): reclaim cached inactive vnode from VFS free listvref(vp): increment reference count on an active vnodevrele(vp): release reference count on a vnode vgone(vp): vnode is no longer valid (file is removed)
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struct vnode {struct mtx v_interlock; /* lock for "i" things */u_long v_iflag; /* i vnode flags (see below) */int v_usecount; /* i ref count of users */long v_numoutput; /* i writes in progress */struct thread *v_vxthread; /* i thread owning VXLOCK */int v_holdcnt; /* i page & buffer references */struct buflists v_cleanblkhd; /* i SORTED clean blocklist */struct buf *v_cleanblkroot;/* i clean buf splay tree */int v_cleanbufcnt; /* i number of clean buffers */struct buflists v_dirtyblkhd; /* i SORTED dirty blocklist */struct buf *v_dirtyblkroot; /* i dirty buf splay tree */int v_dirtybufcnt;
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UCDavis, ecs150Fall 2007 Distributed FSDistributed FS
/
usr sys dev etc bin
/
local adm home lib bin
ftp.cs.ucdavis.edu fs0: /dev/hd0a
Server.yahoo.com fs0: /dev/hd0e
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logical diskslogical disks/
usr sys dev etc bin
/
local adm home lib bin
fs0: /dev/hd0a
fs1: /dev/hd0e
mount -t ufs /dev/hd0e /usr
mount -t nfs 152.1.23.12:/export/cdrom /mnt/cdrom
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UCDavis, ecs150Fall 2007 CorrectnessCorrectness
One-copy Unix Semantics– every modification to every byte of a file has to
be immediately and permanently visible to every client.
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UCDavis, ecs150Fall 2007 CorrectnessCorrectness
One-copy Unix Semantics– every modification to every byte of a file has to
be immediately and permanently visible to every client.
– Conceptually FS sequent access Make sense in a local file system Single processor versus shared memory
Is this necessary?
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UCDavis, ecs150Fall 2007 DFS ArchitectureDFS Architecture
Server– storage for the distributed/shared files.– provides an access interface for the clients.
Client– consumer of the files.– runs applications in a distributed environment.
open closeread writeopendir statreaddir
applications
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UCDavis, ecs150Fall 2007 NFS (SUN, 1985)NFS (SUN, 1985)
Based on RPC (Remote Procedure Call) and XDR (Extended Data Representation)
Server maintains no state– a READ on the server opens, seeks, reads, and closes– a WRITE is similar, but the buffer is flushed to disk before
closing Server crash: client continues to try until server reboots –
no loss Client crashes: client must rebuild its own state – no effect
on server
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RPC - XDRRPC - XDR
RPC: Standard protocol for calling procedures in another machine
Procedure is packaged with authorization and admin info
XDR: standard format for data, because manufacturers of computers cannot agree on byte ordering.
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rpcgenrpcgen
RPC program
rpcgen
RPC client.c RPC server.cRPC.h
datastructure
datastructure
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NFS OperationsNFS Operations
Every operation is independent: server opens file for every operation
File identified by handle -- no state information retained by server
client maintains mount table, v-node, offset in file table etc.
What do these imply???
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Client computer Server computer
UNIXfile
system
NFSclient
NFSserver
UNIXfile
system
Applicationprogram
Applicationprogram
Virtual file systemVirtual file system
Oth
er f
ile s
yste
mUNIX kernel
system calls
NFSprotocol
(remote operations)
UNIX
Operations on local files
Operationson
remote files
*
Applicationprogram
NFSClient
KernelApplicationprogram
NFSClient
Client computer
mount –t nfs home.yahoo.com:/pub/linux /mnt/linux
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UCDavis, ecs150Fall 2007 State-ful vs. State-lessState-ful vs. State-less
A server is fully aware of its clients– does the client have the newest copy?
– what is the offset of an opened file?
– “a session” between a client and a server!
A server is completely unaware of its clients– memory-less: I do not remember you!!
– Just tell me what you want to get (and where).
– I am not responsible for your offset values (the client needs to maintain the state).
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UCDavis, ecs150Fall 2007 The StateThe State
applications
openreadstatlseek
applications
openreadstatlseek
offset
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Network File SharingNetwork File Sharing Server side:
– Rpcbind (portmap)– Mountd - respond to mount requests (sometimes called
rpc.mountd). Relies on several files
– /etc/dfs/dfstab, – /etc/exports, – /etc/netgroup
– nfsd - serves files - actually a call to kernel level code.– lockd – file locking daemon.– statd – manages locks for lockd.– rquotad – manages quotas for exported file systems.
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Network File SharingNetwork File Sharing Client Side
– biod - client side caching daemon
– mount must understand the hostname:directory convention.
– Filesystem entries in /etc/[v]fstab tell the client what filesystems to mount.
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Unix file semanticsUnix file semantics
NFS:– open a file with read-write mode– later, the server’s copy becomes read-only
mode– now, the application tries to write it!!
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Problems with NFSProblems with NFS
Performance not scaleable:– maybe it is OK for a local office.– will be horrible with large scale systems.
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Similar to UNIX file caching for local files:– pages (blocks) from disk are held in a main memory buffer cache until
the space is required for newer pages. Read-ahead and delayed-write optimisations.
– For local files, writes are deferred to next sync event (30 second intervals)
– Works well in local context, where files are always accessed through the local cache, but in the remote case it doesn't offer necessary synchronization guarantees to clients.
NFS v3 servers offers two strategies for updating the disk:– write-through - altered pages are written to disk as soon as they are
received at the server. When a write() RPC returns, the NFS client knows that the page is on the disk.
– delayed commit - pages are held only in the cache until a commit() call is received for the relevant file. This is the default mode used by NFS v3 clients. A commit() is issued by the client whenever a file is closed.
*
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Server caching does nothing to reduce RPC traffic between client and server– further optimisation is essential to reduce server load in large networks– NFS client module caches the results of read, write, getattr, lookup and
readdir operations– synchronization of file contents (one-copy semantics) is not guaranteed
when two or more clients are sharing the same file. Timestamp-based validity check
– reduces inconsistency, but doesn't eliminate it– validity condition for cache entries at the client:
(T - Tc < t) v (Tmclient = Tmserver)– t is configurable (per file) but is typically set to
3 seconds for files and 30 secs. for directories– it remains difficult to write distributed
applications that share files with NFS
*
t freshness guaranteeTc time when cache entry was
last validatedTm time when block was last
updated at serverT current time
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UCDavis, ecs150Fall 2007 AFSAFS
State-ful clients and servers. Caching the files to clients.
– File close ==> check-in the changes. How to maintain consistency?
– Using “Callback” in v2/3 (Valid or Cancelled)
openread
applications
invalidate and re-cache
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Why AFS?Why AFS?
Shared files are infrequently updated Local cache of a few hundred mega bytes
– Now 50~100 giga bytes Unix workload:
– Files are small, Read Operations dominated, sequential access is common, read/written by one user, reference bursts.
– Are these still true?
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Fault Tolerance in AFSFault Tolerance in AFS
a server crashes
a client crashes– check for call-back tokens first.
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Problems with AFSProblems with AFS
Availability what happens if call-back itself is lost??
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GFS – Google File SystemGFS – Google File System
“failures” are norm Multiple-GB files are common Append rather than overwrite
– Random writes are rare Can we relax the consistency?
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UCDavis, ecs150Fall 2007 The MasterThe Master
Maintains all file system metadata.names space, access control info, file to chunk mappings, chunk (including replicas) location, etc.
Periodically communicates with chunkservers in HeartBeat messages to give instructions and check state
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The MasterThe Master
Helps make sophisticated chunk placement and replication decision, using global knowledge
For reading and writing, client contacts Master to get chunk locations, then deals directly with chunkservers
Master is not a bottleneck for reads/writes
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UCDavis, ecs150Fall 2007 ChunkserversChunkservers
Files are broken into chunks. Each chunk has a immutable globally unique 64-bit chunk-handle.
handle is assigned by the master at chunk creation
Chunk size is 64 MB
Each chunk is replicated on 3 (default) servers
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ClientsClients
Linked to apps using the file system API.
Communicates with master and chunkservers for reading and writing
Master interactions only for metadata
Chunkserver interactions for data
Only caches metadata informationData is too large to cache.
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Chunk LocationsChunk Locations
Master does not keep a persistent record of locations of chunks and replicas.
Polls chunkservers at startup, and when new chunkservers join/leave for this.
Stays up to date by controlling placement of new chunks and through HeartBeat messages (when monitoring chunkservers)
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CODACODA
Server Replication:– if one server goes down, I can get another.
Disconnected Operation:– if all go down, I will use my own cache.
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Disconnected OperationDisconnected Operation
Continue critical work when that repository is inaccessible.
Key idea: caching data.– Performance– Availability
Server Replication
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ConsistencyConsistency
If John update file X on server A and Mary read file X on server B….
Read-one & Write-all
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UCDavis, ecs150Fall 2007 Read x & Write (N-x+1)Read x & Write (N-x+1)
read
write