sluggish disk performance.

bsdml at werner.st bsdml at werner.st
Mon Aug 21 17:25:15 UTC 2006


Hi
>> In the last episode (Aug 21), Chris Knipe said:
>>> Disks   ad0                               ofod            intrn
>>> KB/t  16.83                               %slo-z    35456 buf
>>> tps     103                             4 tfree       414 dirtybuf

well, with 100% random I/O at 7200rpm (or 120rpm per second = maximum
number of theoretical IOPS) that is a typical number I see everywhere.

The 16KB average I/O-Size are OK but could be higher - but definitely your
I/O-Pattern an not the avg. transactions size are the cause of your
1.7MB/sec

>>> MB/s   1.70                                         20988 desiredvnodes
>>> % busy   98                                          5247 numvnodes
>>>                                                     4223 freevnodes
>>>
>>> Got 1 ATA100 Seagate 120GB disk in there at the moment.... 1.7MB/s at
>>> 98% busy?  Surely, that figure is WAY low???  I'd expect atleast
>>> about 10MB/s on ATA100.
>>
>> That number's about right for random I/O and small blocksizes, which is
>> what the KB/t field shows.  If you were doing sequential I/O, the KB/t
>> field would be at or near 128.  Are you also running a "du", "cvs
>> update", or other command likely to be doing random disk accesses?
>
> ALTER TABLE on a 200MB mySQL table?
>
> I guess its time for a dedicated disk then....

Depends on the number of random I/O's you want to be able to handle -
could be that you might want 2 or more aswell :-)

Cheers, Martin



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