[Gllug] Disk performance

[Alain Williams]

A system that I look after the dozen or so users report occasional 'slowdowns' of several seconds
where there will be little response. The work is stock control/invoicing type app written using C-ISAM
databases. 8GB RAM, 2 dual core AMD opterons.

The disks seem to be the bottle neck. If I pick a bad time I show iostat & dstat output below.
I have straced a few applications and not seen fsync() so there is no real excuse for slowing
down on write - there is plenty of RAM for buffer cache.

I wonder if increasing the value in /sys/block/sda/device/queue_depth (currently 31) would help.
queue_type contains 'simple' - worth tweaking ?

iostat output:

Time: 11:10:07
Device:         rrqm/s   wrqm/s   r/s   w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00    62.00  0.00 369.00     0.00  3456.00     9.37   131.06  343.04   2.71 100.10
sdb               0.00    53.00  0.00 417.00     0.00  3912.00     9.38    90.88  219.61   2.40 100.10
sdc               0.00   106.00  0.00 344.00     0.00  3800.00    11.05   144.33  402.43   2.91 100.10

Time: 11:10:08
Device:         rrqm/s   wrqm/s   r/s   w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00     6.00  0.00 354.00     0.00  3000.00     8.47   136.61  417.13   2.83 100.10
sdb               0.00     5.00  0.00 368.00     0.00  3008.00     8.17    50.60  156.20   2.72 100.10
sdc               0.00    10.00  0.00 328.00     0.00  2864.00     8.73   140.25  417.20   3.05 100.10

Time: 11:10:10
Device:         rrqm/s   wrqm/s   r/s   w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00    24.55  0.00 178.57     0.00  1639.29     9.18    59.50  406.08   3.73  66.56
sdb               0.00    24.11  0.00 138.84     0.00  1310.71     9.44    11.31   89.51   2.68  37.23
sdc               0.00    25.45  0.00 180.36     0.00  1660.71     9.21    55.93  404.14   3.10  55.98


dstat:

-----time----- --dsk/sda-- --dsk/sdb-- --dsk/sdc-- ----total-cpu-usage---- ----------------interrupts--------------- ---load-avg--- ---procs--- ---system--
  date/time   | read  writ: read  writ: read  writ|usr sys idl wai hiq siq|  1     4     12    50   169   209   225 | 1m   5m  15m |run blk new| int   csw

02-11 11:10:07|   0  1700k:   0  1968k:   0  1880k|  0   0  27  72   1   0|   0     0     0    35     0  2296  1089 | 0.6    1  0.7|  0   4   0|4423   213
02-11 11:10:08|   0  1436k:   0  1492k:   0  1416k|  6   3  25  65   0   1|   0     0     0    20     0  2071  1006 | 0.6    1  0.7|  1   6   0|4096   145
02-11 11:10:10|   0  1752k:   0  1360k:   0  1736k|  1   1  50  48   0   0|   0     0     0    47     0  1965  1137 | 1.1  1.1  0.7|  0   0   0|5317   353



The 3 disks are set up as RAID 1 (ie a 3 way mirror - partly due to history of disc failure due to SATA speed
negotiation errors [resulting in too high a speed] that meant data timeouts -- all now fixed).

-- 
Alain Williams
Linux/GNU Consultant - Mail systems, Web sites, Networking, Programmer, IT Lecturer.
+44 (0) 787 668 0256  http://www.phcomp.co.uk/
Parliament Hill Computers Ltd. Registration Information: http://www.phcomp.co.uk/contact.php
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