Transcript Multimedia File Systems

MM File Management
Karrie Karahlaios and Brian P. Bailey
Spring 2007
Announcements
Physical Disk Structure
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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
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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
Pi = 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
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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   T
to serve real-time clients
– Use (1   )  T
to serve non real-time clients
• To retrieve T*ri bits for each client, the
controller must ensure time to retrieve
T*r1, …, T*rn bits does not exceed   T
Number of Disk Blocks
• If b is block size, then maximum number
of disk blocks to be retrieved for ri is
 T  ri 
 b 
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
 T  ri 
2  tseek  ( 
)  (trot tsettle)

i 1  b 
q
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
T  ri
rdisk
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
T  ri
T  ri 
2  tseek  ( 
)  (trot tsettle)  
  T

i 1  b 
i 1 rdisk
q
q
• Admit new client, if on adding it, this
equation is still satisfied
Admission for Non RT Clients
• Remainder of the period (1   )  T is for
requests from non real-time clients
• Let di be the data requested from Ci
• Number of blocks is  di 
b
 
Admission for Non RT Clients
• For each request, latency plus transfer time is
di
 di 
 b   (trot tsettle)  rdisk
 
• Over all requests p, this becomes
p
di
 di 

 b   (trot tsettle)  rdisk  (1   ) * T
i 1
i 1
p
• Admit new non RT client, if on adding it, above
equation is still satisfied
Example
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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?