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Mike McCarty 02-12-2010 09:52 AM

Help Diagnose Slow Disc Access
My machine has been running slower and slower, and top seems
to indicate lots of I/O wait. I have two ATA discs on a single
cable, wired for cable select. The master is much faster than
the slave, which seems to indicate a hardware, possibly disc,

Trimmed output:

$ mount
/dev/hda5 on / type ext3 (rw)
/dev/hdb1 on /home type ext3 (rw)
/dev/hda3 on /boot type ext3 (rw)

Here's the master, and faster, disc:

$ sudo /sbin/fdisk -l /dev/hda

Disk /dev/hda: 40.0 GB, 40020664320 bytes
16 heads, 63 sectors/track, 77545 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

Device Boot Start End Blocks Id System
/dev/hda1 1 8625 4346968+ b W95 FAT32
Partition 1 does not end on cylinder boundary.
/dev/hda2 * 8626 60915 26354160 7 HPFS/NTFS
Partition 2 does not end on cylinder boundary.
/dev/hda3 60916 61118 102312 83 Linux
Partition 3 does not end on cylinder boundary.
/dev/hda4 61119 77545 8279208 f W95 Ext'd (LBA)
Partition 4 does not end on cylinder boundary.
/dev/hda5 61119 76505 7755016+ 83 Linux
/dev/hda6 76506 77545 524128+ 82 Linux swap

Here's the slave, and slower, disc:

$ sudo /sbin/fdisk -l /dev/hdb

Disk /dev/hdb: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/hdb1 1 38913 312568641 83 Linux

Here are some timings on an otherwise completely
unloaded machine:

$ ls -l *.iso /tmp/*.iso
-rw-rw-r-- 1 jmccarty jmccarty 724559872 Nov 17 22:23
-rw-rw-r-- 1 jmccarty jmccarty 724559872 Feb 5 17:30

$ time cp KNOPPIX_V6.2CD-2009-11-18-EN.iso fred.iso

real 16m19.492s
user 0m0.169s
sys 0m5.461s

$ time cp /tmp/KNOPPIX_V6.2CD-2009-11-18-EN.iso /tmp/fred.iso

real 1m12.639s
user 0m0.165s
sys 0m5.414s

All discs pass fsck with no problems, though the larger (slower)
disc takes an extremely long time to do so.

I tried using smartctl and it doesn't report any anomalies that
I can see, though when I tried running the self tests, the "fast"
disc passed, but the "slow" disc didn't complete the test in a
reasonable amount of time (over 1/2 hour) each time I tried.
This was done with all file systems mounted.

I have shut down, powered down, and unplugged, then reseated
the all the ATA cable connectors, and reseated all RAM, and
the add-on video board etc., with no change to the timings.

Can anyone give me advice on how to proceed with diagnostics?
What diagnostics does Linux provide which can help me figure
out what's going on? Is there a way to get statistics on
missing interrupts, for example? Perhaps the disc is not generating,
or the cable is dropping, IRQs, and the kernel is falling back
to polling, for example. That's the kind of information I'm
looking for. Something to point me in the direction of where
I may be having a hardware problem.

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