MM File Management Karrie Karahlaios and Brian P. Bailey Spring 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 generally P i = k / i α 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(P i )=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 –D i = D i + f(N i ); where f(N i ) = N i / N max –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 r i Retrieval rate for each client must be > T*r i Ensure that the file system in each period T can retrieve T*r i 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*r i bits for each client, the controller must ensure time to retrieve T*r 1, …, T*r n bits does not exceed

Number of Disk Blocks If b is block size, then maximum number of disk blocks to be retrieved for r i is

Latency Retrieval of a disk block involves a seek to the track containing the block, a settle time delay, and a rotational delay Let t seek, t rot, and t settle be the worst case times for each measure

Maximum Latency Thus, the maximum latency for servicing clients r 1, r 2, …, r q is

Transfer Time If the transfer rate from the innermost track of the disk is r disk, then the time to transfer T*r i bits of data for request r i is

Admission for Real-Time Clients Thus, the total time to retrieve T*r 1, …, T*r q bits for requests R 1, …, R q is the sum of the latency and transfer times Admit new client, if on adding it, this equation is still satisfied

Admission for Non RT Clients Remainder of the period is for requests from non real-time clients Let d i be the data requested from C i Number of blocks is

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

Example Transfer rate (r disk ) = 100 KB / sec Cycle time (T) = 10ms Max latency = 1ms Client A data rate (r 1 ) = 45 KB/sec Client B data rate (r 2 ) = 40 KB/sec Are the two real-time clients admissible? If so, what proportion of the cycle time is needed to serve these clients?