Where did the seek time go? And rotational latency?
[I hope my calculations are correct]
This is mine (a IBM Deskstar 75GXP)
Sustained data rate (MB/sec) 37
Seek time (read typical)
Average (ms) 8.5
Track-to-track (ms) 1.2
Full-track (ms) 15.0
Data buffer 2 MB
Latency (calculated 7200 RPM) 4.2 ms
So sustained data rate is 37 MB/s
> hdparm -t gives around 35 MB/s
best I got during testing or real files is 32 MB/s
A small calculation:
Track-to-track time is 1.2 ms + time to rotate 4.2 ms = 5.4 ms
In this time I can read 37 MB/s * 5.4 ms = 200 kB or
more than 48 pages (instead of moving the head to the closest
track I could read 48 pages...)
Average is 8.5 ms + 4.2 ms => 114 pages
Reading a maximum of 114 pages at a time gives on the average
half the maximum sustained throughput for this disk.
But the buffer holds 512 pages... => I tried with that. [overkill]
* Data from a Seagate Cheetah X15 ST336732LC (Better than most
disks out there - way better than mine)
Formatted Int Transfer Rate (min) 51 MBytes/sec
Formatted Int Transfer Rate (max) 69 MBytes/sec
Average Seek Time, Read 3.7 msec typical
Track-to-Track Seek, Read 0.5 msec typical
Average Latency 2 msec
Default Buffer (cache) Size 8,192 Kbytes
Spindle Speed 15K RPM
Same calculation:
Track-to-track: (2 + 0.5) ms * 51 MB = 127 kB or 31 pages (42 pages max)
Average: (2 + 3.7 ms) * 51 MB/s = 290 kB or almost 71 pages (96 pages max)
Reading a maximum of 71 pages gives on the average half the
maximum sustained throughput. [smart buffering in the drive might help
when reading from several streams]
/RogerL
-- Roger Larsson Skellefteċ Sweden - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/