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Multimedia Storage Issues

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Media vs. Documents

large file sizewrite once, read manydeadlines!

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OS Review: Disk

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Disk

head, spindle, track, sector, cylinderseek time, rotational latency

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Disk Scheduler

disksched

I/O Request

read/write command

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Disk Scheduling Algorithm

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Best Effort SchedulerFCFS: First come first serveSSTF: Shortest seek time firstSCAN: Serve all requests in one

direction, then in the other direction

C-SCAN: Serve all requests in one direction only, then “jump” back to the other end and repeat.

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Other Schemes EDF: Earliest deadline first EDF-SCAN: Use SCAN to schedule

request with same deadline. FD-SCAN: SCAN towards direction

with earliest feasible deadline SSEDO: Rank request by deadline and

use formula (seektime*rank) SSEDV: Use formula (lifetime* +

seektime*(1- )

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Data Placement

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Contiguous

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Fragmented

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Stripping (RAID-0)

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Mirroring (RAID-1)

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Parity (RAID-5)

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Parity

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Summary of Concerns

High ThroughputFault TolerantLoad balancing

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Design Parameters

unit of striping (block size)degree of striping (num of disks)

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Stripe Unit

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Stripe Unit

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Effects of Block Size

Block Size

servicetime

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Problem

Find block size such that the service time for the most heavily loaded disk is minimize

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Analysis Model: A Plan

service time of the busiest disk as a function of block size

expected num of blocks accessed on the busiest disk

expected num of blocks accessed on any disk

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Assumptions

Non-redundant arrayAnalyze read operation only

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Variables Declaration

Nd: Number of DisksNc: Number of ClientsB : Block sizeNb(i,j): Number of blocks client i

access from disk jNb(i,*): Total number of blocks

client i access

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Analysis

Suppose i request m blocks

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Reading 1 block

Prob(Nb(i,j)=1)

1

1

1

)),((

)),((

D

D

N

m D

DDb

N

m Db

NmNmNjiNP

NmmjiNP

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Reading k block

Prob(Nb(i,j)=k)

1

1

1

)),((

))1(),((

D

D

N

m D

DDb

N

m DDb

NmNmkNjiNP

NmmNkjiNP

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Total Num of Blocks from Disk jNb(*,j) =

CN

ib jiN

1

),(

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Busiest Disk

Nmax = )(*,max..0

jNDNj

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Access Time

T = Nmax(ts + tr + Btt)

Depends on:block size Bdisk characteristic ts,tr,tt

server design Nd

workload characteristic Nc, N(i,j)_

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Model Verification

ServiceTime

Block Size

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Imbalance and Overhead I = 1 – Tavg/Tmax

O = 1 – NmaxBTt/Tmax

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Effects of Block Size

Block Size

Imbalance

Overhead

~optimum

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Effects of Clients

Block Size

Imbalance

Overhead

Nc incr, I decr, opt B incr

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Effects of Clients

BlockSize

Nc

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Effects of Num of Disks

Nd incr, N(i,j) decr, I incr, opt B decrBlockSize

Nd

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Choosing Block Size

Given Nc, assume rest is fixed find B that minimize T

if T < duration of a round incr Nc and try again

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There is more..

redundant arrayfinding optimum degree of

striping

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Cello: Disk Scheduler

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Problem

How to co-exist with other applications?

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Classes of ApplicationsReal-time

hard/softperiodic/aperiodic

Best-effortinteractivetroughput-intensive

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Attempt 1

Priority-based schedulerAlways schedule real-time tasks

ahead of best-effort tasks

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Attempt 2

Partition time slots into real-time and best-effort

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Attempt 3

Assign weight to application class, based on their priority

Service based on weight

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Attempt 3: Cello

Two-level scheduling

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Class Independent Scheduler

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Allocating Disk BandwidthAllocate in proportion to timeAllocate in proportion to bytes

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Notations

P : Interval of a roundI : Total idle time so far in this

roundUi : Time allocated so far to class

iwi : Weight for class iW : Total weight for all class

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Proportionate Time-Allocation

Ui < P-I

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Proportional Time Allocation

Ui < (P-I)wi/W

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Example

Weights 1:1:2P = 100I = 30

10 20 16 14

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Pick a class i which is underutilized, i.e.

Ui < (P-I)wi/W

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r

next

prev

Class specific schedule will return a request to be inserted,plus to location in the scheduled queue for insertion.(r, prev, next)

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Need to Make Sure..

Does not exceed share for class i

Ui + T < (P-I)wi/W

service time for new request

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Need to Make Sure..

Total service time does not exceed available time

Ui + T + T’next – Tnext < P-I

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Update and Repeat

next

prev

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What if..

a class have no pending request?put in no_work group

a class violates one of the constraints?put in too_much_work group

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Improving Utilization

When disk is idle, distribute service time among classes in too_much_work group

Pick a request from this class and put into scheduled queue

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Free Time Utilization

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Class Specific SchedulerHow to pick (r, prev, next)?

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Slack Time

li = Latest time you must begin servicing request i

ei = Earliest time you may begin servicing request i

di = Deadline for request isi = Slack time for i = li - ei

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Slack Time

li = min(di, li+1) – Ti

ei = ei-1 + Ti-1

Special case when request is at the beginning of queue or end of queue.

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Slack Time

i

i

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Class 1: Interactive Best Effort

next

prev

pick r at front of pending queue, insert into earliest positionin scheduled queue without violating deadlines of others.

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Class 2: High Throughput Best Effort

next

prev

pick r at front of pending queue, insert into tail of scheduled queue. Order consecutive best effort request by SCAN order.

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Class 3: Real Time Application

next

prev

pick r in EDF order. Insert into scheduled queue such that it will meet its deadline and will not violate deadlines of others

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Evaluationsresponsetimeof besteffortapp

# video

SCAN

Cello

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Evaluations%bandwidth

time

50%

No video

text app

video app

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Cello Summary

OS disk schedulerSupport multiple classesProtect classes from each otherWork conservingUse two-level scheduling