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Do I have an Ultra160 Disk Bottleneck?

Posted on 2004-04-30
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Last Modified: 2010-04-03
Hi all,

I've been trying to determine if I indeed have an Ultra160 disk bottleneck on our system.  The system is a Netfinity 7100 running Win2k.  I've got 3 RAID 5 arrays and 1 mirrored array.  When running performance monitor during disk access slow downs, I notice that the disk queue length passes from 0 to more than 50 as soon as the total "disk byte/sec " counter reaches 40 MB/sec.  It doesn't matter if the 40 MB/sec is reached on indivudual arrays or for all the arrays combined. Unless I am dead wrong, this is well below the theoretical 160 MB/sec that Ultra 160 is supposed to deliver.

Does that indicate I have a disk subsystem bottleneck?  Is there something else I am missing or should check?  

Thanks in advance.
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Question by:TechnoBuzz
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Callandor earned 125 total points
ID: 10964220
That 160 number is rarely reached, and only if the disk cache is being used, which empties out in a matter of seconds.  80MB/sec is a more realistic number for maximum throughput on the outside cylinders with everything else optimized.  A tool like HDTach will tell you what your array will deliver.
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by:Phill_upson
ID: 10964772
Also depends on the quality of your RAID controllers and other disks on each chain.  One cable supporting 2 hard disks might be capable of 160MB/sec theoretically, but if you have a UDMA 66 disk on the same chain, access will be held up by that disk.  Again, always worth checking the quality of your cable, bad or cheap cables can cause problems of their own.

From the HDTach reading (tool mentioned above) if you have a spare machine, try putting the disk in there on its own port and HDTach it again, that way you can try and identify if the disk is the issue and contact the manufacturer.
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Expert Comment

by:redking44
ID: 11010008
I have not played RAID so i can only comment on SCSI in general

it's interesting that you top out at 40 MB/sec - which is the SCSI-UW spec. ALmost any other speed and i'd say it ws a bottleneck of some sort,  but this is a stanbdard SCSI speed so i'm sure it's related.

I'd make sure the cables were really U-160 cables, and not UW cables. This may not be easy because they look identical.  I'd also check the setup in the SCSI card(s) - they can be set to work at any speed you choose, and might be throttled back to 40MB/sec..

i suppose...  are your disks really u160  drives?
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by:Callandor
ID: 11372630
TechnoBuzz,

Have your questions been answered?  We haven't heard from you, so why the C grade?
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by:TechnoBuzz
ID: 11397266
Thanks for the link.

Considering this, please change the grade to a "B", since it is after all more approriate.

Thanks Callandor for your input.
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by:Callandor
ID: 11398560
Thanks for looking over the grading guidelines.
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