September 15, 2003 at 8:44 pm
hi!
I am monitoring the peak times on our sql server system. I set up a performance counter log to monitor raid 5 for an hour and all the values are similar to the values below:
Physical Disk %Disk Time: 220.9801458
Physical Disk %Avg Disk Queue Length:
2.209801458
Do I need to divide the disk time and the disk queue length by 5 to get the numbers for each disk in the array ?
Do I consider the above values very high since %Disk Time > 90 and disk queue > 2
thanks in advance
September 16, 2003 at 3:56 am
Within your operating system you cannot see your physical drives individually.
Your RAID5 disk are consideres as a whole for it. Therefore you do not have to divide your values by 5.
Bye
Gabor
Bye
Gabor
September 24, 2003 at 6:51 am
quote:
Do I consider the above values very high since ... disk queue > 2
As for disk queue, the value is high when it is higher then 2 x [number of phisical disks in array]
September 28, 2003 at 6:26 am
If you are using hardware raid you should take the total number of disks into account when looking at those numbers.
Also note that 'Queue lenght' and '% disk time' are exactly the same figures: 100% busy and a queue lenght of 1, mean that at the time permon sampled the disk was busy servicing an I/O without requests waiting.
A number of 2 or 200% thus means one servicing and one waiting if you have 1 disk in your hardware raidset.
If you have 5 disks in this set it means that 3 are doing nothing.
If this is software raid (does anybody still use this??), then the above will be the case: then you are looking at one disk with one I/O busy and 1 waiting.
Note that these counters are snapshots, so it's not an average between two intervals..
This doesn't mean that a constantly high disk queue is not a problem though.
(indeed the value of 2 per disk is used: when having a raid set of 5 disks a value of 10 or less is acceptable ,a constantly higher value *can* be a (a sign of a) problem
What I think is more important are the average read and write times. If they are up significantly you are in trouble..I'm not happy with I/O avgs of >10ms per I/O..
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