MM File Management

Karrie Karahlaios and Brian P. BaileySpring 2007

Announcements

Physical Disk Structure

• Sector• Track• Platter• Cylinder• R/W head

• Example– 16 heads x 1400 cyls x

16 sectors/track x 512 bytes/sector = 183.5MB

Measures of Performance

• Seek time (ms)– time to move disk arm

to a specific track

• Latency (ms)– time for sector to

rotate under disk arm

• Transfer rate (Mbps)– data that can be read

in one time unit

Zoned Bit Recording

• Utilize larger, outer tracks– early disks could not handle

varying number of sectors / track– reduce density of outer sectors

• Each zone (set of tracks) has variable number of sectors– outer part can hold more data

and support higher transfer rates

File System

• Mapped onto physical disk structure– want to match user’s conceptual model

• Collection of files and directories– file is logical storage unit– directories contain information about files

(names, type, location, size, protection, etc.)• Basic operations

– create, write, read, reposition, delete– sequential and random access

Allocation Methods

• Contiguous

• Linked

• Constrained

• Striping

• … and many others

Continuous

• Occupy contiguous set of blocks

• Strengths– minimizes seek time– supports sequential and random access

• Weaknesses– suffers external fragmentation

Linked

• Stored as a linked list of blocks

• Strengths– eliminates external fragmentation– supports files of arbitrary length

• Weaknesses– random access slow, overhead of pointers– susceptible to block errors

Constrained

• Linked structure, but allocate next block based on “distance” from previous one– distance = predicted seek and latency

• Strengths– improves sequential access– minimizes seek time

• Weaknesses– increases algorithm complexity

Striping (RAID-0)

• Stripe file across an array of N disks– divide file into stripes, dive stripe into units,

assign each unit to different disk

• Strengths– reduces disk access time by N

• Weaknesses– susceptible to failure of any one disk – p(failure) = N * p(any one disk failing)

MM File System Requirements

• Storing/retrieving multimedia files – large size; continuous periodic requests

• Maintain high throughput

• Support RT and non RT requests

• Guarantee a sustained level of service

Meeting the Requirements

• Methods of placing data on disk

• Scheduling algorithms

• Admission control policies

• Maximize transfer time

Zipfs Law

• Probability of occurrence of the kth most common word is proportional to 1/k– applies to many observable events

• More generallyPi = k / iα where

– i is the ith most popular item; k is a constant; alpha is close to 1

Apply to File Allocation

• For multimedia, assume that– alpha=1

– Sum(Pi)=1

• Compute the probability of each multimedia file being accessed– use for layout and prefetching

Scheduling Algorithms

• FCFS

• SSTF

• SCAN and C-SCAN

• EDF

• SCAN-EDF

• Understand each algorithm and weigh advantages and disadvantages

FCFS

• Serve requests based on incoming order

• Inherently fair

• Does not consider location of requests– can lead to high overhead

SSTF

• Select request closest to current position– minimizes seek time/overhead

• May cause starvation of some requests

SCAN and C-SCAN

• Serves all requests in current direction– reverses when no more requests– serves middle tracks better than edges

• C-SCAN scans across disk in cycles– more fair to the edge tracks

EDF

• Attach deadlines to each request– select request with earliest deadline– can have high overhead

SCAN-EDF

• SCAN-EDF selects – earliest deadline, or if same deadline– select request closest to the disk’s center

• Use EDF, but perturb deadlines– Di = Di + f(Ni); where f(Ni) = Ni / Nmax

– Consider direction?

Admission Control

• Based on the admission control policy discussed in the paper:

– C. Martin, P.S. Narayan, B. Ozden, R. Rastogi, and A. Silberschatz. The Fellini Multimedia Storage System, Journal of Digital Libraries, 1997.

Mathematical Setup

• Client requests received in cycles of duration T– T is referred to as the common period of the system– assumes circular (C-SCAN) scan of the disk

– consumption rate of each real-time client is ri

• Retrieval rate for each client must be > T*ri

• Ensure that the file system in each period T can retrieve T*ri bits for each client

Setup (cont.)

• Serve both real and non-real time clients

• Serve real-time clients using fraction of T– Use to serve real-time clients– Use to serve non real-time clients

• To retrieve T*ri bits for each client, the controller must ensure time to retrieveT*r1, …, T*rn bits does not exceed

TT )1(

T

Number of Disk Blocks

• If b is block size, then maximum number of disk blocks to be retrieved for ri is

b

rT i

Latency

• Retrieval of a disk block involves a seek to the track containing the block, a settle time delay, and a rotational delay

• Let tseek, trot, and tsettle be the worst case times for each measure

Maximum Latency

• Thus, the maximum latency for servicing clients r1, r2, …, rq is

q

i

settleroti

seek ttb

rTt

1

)()(2

Transfer Time

• If the transfer rate from the innermost track of the disk is rdisk, then the time to transfer T*ri bits of data for request ri is

disk

i

r

rT

Admission for Real-Time Clients

• Thus, the total time to retrieve T*r1, …, T*rq bits for requests R1, …, Rq is the sum of the latency and transfer times

• Admit new client, if on adding it, this equation is still satisfied

Tr

rTtt

b

rTt

q

i disk

iq

i

settleroti

seek

11

)()(2

Admission for Non RT Clients

• Remainder of the period is for requests from non real-time clients

• Let di be the data requested from Ci

• Number of blocks is

T )1(

b

di

Admission for Non RT Clients

• For each request, latency plus transfer time is

• Over all requests p, this becomes

• Admit new non RT client, if on adding it, above equation is still satisfied

disk

isettlerot

i

r

dtt

b

d

)(

p

i

p

i disk

isettlerot

iT

r

dtt

b

d

1 1

*)1()(

Example

• Transfer rate (rdisk) = 100 KB / sec

• Cycle time (T) = 10ms

• Max latency = 1ms

• Client A data rate (r1) = 45 KB/sec

• Client B data rate (r2) = 40 KB/sec

• Are the two real-time clients admissible?

• If so, what proportion of the cycle time is needed to serve these clients?