Spotlight Series

WAFL Overviewhttp://www.netapp.com/tech_library/3002.html

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WAFL – No pre-allocated locations (data and metadata blocks are treated equally). Writes go to nearest available free block.

Writing to nearest available free block reduces disk seeking (the #1 performance challenge when using disks).

WAFL: Write Anywhere File LayoutFilesystem for Improved Productivity

Berkeley Fast File System/Veritas File System/NTFS/etc. – Writes to pre-allocated locations (data vs. metadata)

...

1-2 MB Cylinders

...

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WAFL uses integrated RAID4

RAID4 is similar to better known RAID5:

– RAID5: parity is distributed across all disks in the RAID group

– RAID4: parity is contained in a single disk in the RAID group

Tradeoffs with the single parity disk RAID model:

– PRO: The RAID group can be instantly expanded by adding (pre-formatted) data disks.

– CON: The parity disk is perceived to be the ‘hot spot’ in the RAID group, due to intensive XOR parity calculations on it.

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WAFL eliminates the parity hot spot

WAFL overcomes the ‘classic’ parity-disk hotspot issue, by the use of flexible write allocation policies:

– Writes any filesystem block to any disk location (data and meta data)*

– New data does not overwrite old data

– Allocates disk space for many client-write operations at once in a single new RAID-stripe write (no parity re-calculations)

– Writes to stripes that are near each other

– Writes blocks to disk in any order

* except root inode

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Result: Minimal seeks and no hotspot

file1

file3

file2

Typical File System

Long head seeks

especiallyon

paritydisk

file1

file3file2

WAFL

Shorthead seeksacross

all disks

1 file at a time Multiple files at once

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WAFL Combined with NVRAM

WAFL uses NVRAM “consistency points” (NetApp’s flavor of journalling), thus assuring filesystem integrity and fast reboots.

CP flush to disk occurs once every 10 seconds or when NVRAM reaches half full.

NVRAM placement is at the file system operation level, not at the (more typical) block level. This assures self-consistent CP flushes to disk.

No fsck!

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NVRAM

General-purpose NV-RAM

Disk Driver

SemanticWrite Alloc

NFS or CIFS

TCP/ or UDP/IP

NetAppNV-RAM

File System

Disk Driver

NVRAMSemanticWrite Alloc

TCP/ or UDP/IP

File System

NVRAM safe-storesthe disk blocks

NVRAM safe-stores the FS operation

NVRAM placement is key!

NFS or CIFS

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Seek Example in a SAN environment

Assume 4K disk blocks and 5 msec for one seek+rotate

100MB/sec FC bandwidth x .005sec = .5MB worth of data blocks not sent on the FC channel during that seek

.5MB x 1 block/4KB = 128 blocks not sent

Therefore a 5ms seek for just 1 block equates to a 128 block penalty

Conclusion: one seek every 128 blocks or less ( ~1%) wastes at least half of your FC bandwidth!

(seek 1 block)128 blocks 128 blocks (seek 1 block)

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The protocol overhead issue

• Yes, we have TCP/IP overhead.

• Yes, we have double-buffering overhead.

• Yes, we might well have <obscure performance gotcha>.

• Despite all that, we're able to improve performance, even

with databases (now over 40% of NetApp customer base).

• Clearly, we're doing *something* sufficiently right to

make up for the overhead.

Isn’t NAS slower than local disk?

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The protocol overhead issue

• TCP/IP might seem to be a massive overhead, but passing

packets up and down the stack turn out only to consume

microseconds per request.

(For example: 1Ghz CPU speed == 1 ____second clock cycle.

Keep the timing in perspective with today’s CPU speeds!

• TCP/IP might seem to be a massive overhead, but passing

packets up and down the stack turn out only to consume

microseconds per request.

(For example: 1Ghz CPU speed == 1 nanosecond clock cycle. So 1000 extra

CPU cycles for TCP stack = 1000x1ns = 1 microsecond)

• Eliminating head seeks, which WAFL does better than any

other file system thanks to its full integration with RAID,

saves whole milliseconds, eg, a 1000x savings.

(5ms seek)128 blocks 128 blocks (5ms seek)

TCPoverhead

TCP overhead is small by comparison

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NetApp FilersHigh speed, low latency

SFS97_R1 Performance - NFS v3 TCP http://www.spec.org/cgi-bin/osgresults?conf=sfs97r1

F825F825 F880F880FAS960cFAS960c

• RAID protected• Single file system

• RAID protected• Single file system

FAS960FAS960FAS940FAS940

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Summary

WAFL extracts more ops/sec from a single drive due to minimum seeks.

More ops/sec equates to faster overall performance WAFL’s “anywhere” property makes NetApp’s RAID-4 the performance and scalability winner.

Fastest File System in the world with RAID enabled