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Subsections
Main issues:
- performance,
- space usage.
Main considerations:
- file sizes,
- file usage patterns,
- memory vs. disk speeds,
- disk characteristics.
In teaching/programming envs, files tend to be[ODCH$^+$85]:
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
File size distribution in CSE ``VaST''
network[Elp93].
- 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
- 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: The bad old days
Up: 09-fs
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Gernot Heiser
2002-10-11
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