[CSE]  Advanced Operating Systems 
 COMP9242 2002/S2 
UNSW

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Subsections

File Systems




Main issues:
  • performance,
  • space usage.
Main considerations:
  • file sizes,
  • file usage patterns,
  • memory vs. disk speeds,
  • disk characteristics.

File sizes and usage patterns:

In teaching/programming envs, files tend to be[ODCH$^+$85]:

  • small (\(>\)50% \(<\)2kB, average size 22kB),
    • But growing -- (1992 study average sizes \(>\) than 1985 study)

  • short lived,
  • open for very short times,
  • mostly read/written entirely,
  • mostly processed sequentially,
  • many more reads than writes (``worm'': write once, read many),
  • but[RLA00] the most-accessed files are written many times more than read (logs, mail queue files)
  • caching can eliminate most reads.

But in a commercial transaction-oriented environment they tend to be[RBK92]:

  • larger
  • long lived
  • open for long periods of time
  • mostly read/written a record at a time
  • shared between several processes
  • mostly processed sequentially
  • Still 3 to 4 times as many reads as writes

dist

File size distribution in CSE ``VaST'' network[Elp93].

Disk characteristics


  • Unit of access is sector, order of kB.
  • A number of sectors per track,
  • rotational delay between non-consecutive sectors.
  • Several platters, corresponding tracks form cylinders.
  • Head movement required between cylinders - seek delay.
  • Seek times are order of ms.
  • Maximum throughput is of the order of MB/s.
  • Time to read/write single sector is dominated by seek time.

But ...
  • Modern discs, raid, etc., hide this from the filesystem.
  • Accessing logically adjacent blocks (probably) faster than random
  • Failing discs remap sectors to spare tracks

IBM Ultrastar Fiberchannel

Sector size
512bytes
Nr. heads
12
Rotational speed
10 000rpm
Sectors/track
varies from 440 (zone 14) to 864 (zone 0)
Number of Cylinders
36 735
Seek time
4.7ms (read) 5.9ms (Write); typical.
Seek time (full stroke)
11ms
Latency
3ms (ave) (based on 6ms for one rotation == 10 000rpm)
Media transfer rate
73.7 (outermost zone) to 37.5 (innermost) MB/s
Interface transfer rate
200MBytes/sec (2G fiberchannel)
Sustained data rate
66.7MB/s (typical)


next up previous
Next: The bad old days Up: 09-fs Previous: 09-fs
Gernot Heiser 2002-10-11