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Old 01-16-2006, 10:28 PM   #1
ludwigs3rd
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Software Raid: High Cpu Utilization


Hi all,
I might be mistaken but I think after reading about software RAID, CPU utilization for a RAID1 array with two 80GB drives on a PIII 700Mhz with 512MB RAM weren't supposed to result in a load of 3+. Everytime I use the RAID md0_RAID1 pops up with about 10% CPU and if I'm copying lots of files %CPU usage is around 70%! I have the 2 drives in the RAID array on the same card, but each are master on their own IDE port. The card I'm using is a Promise Ultra66 ide controller.
Note: I know about the Ultra66 hack to make it a Fastrak66 card but don't want to do any of that, so please no postings on that.
Previously I had messages in /var/log/messages as follows:
Jan 13 20:31:56 linux kernel: hde: dma_intr: status=0x51 { DriveReady SeekComplete Error }
Jan 13 20:31:56 linux kernel: hde: dma_intr: error=0x84 { DriveStatusError BadCRC }

It turns out that it had to do with the DMA settings on the drive. I used the IDE DMA tool in Suse and changed it to OFF but performance was really slow. These are two IBM/Hitachi drives so I used the tools available on their website to confirm the drives are in perfect health. I also low-level formated both of them to see if that fixed the error. So I think currently the drives are set to UltraDMA 33 (mode 2 I believe) and that seems to fix the error (I also get no error with UDMA OFF). I'd like to get it to UDMA66 but if I recall I see the same error above in the logs if I do that. So my goals are to 1) fix my CPU utilization issue if possible 2) get both drives using UDMA66. I appreciate any suggestions or help anyone can offer. Thanks to everyone in advance.
 
Old 01-17-2006, 07:13 AM   #2
Brian Knoblauch
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I'm not surprised to see that kind of load with that processor and that setup. Software RAID tends to chew up processor cycles. As do ATA drives (even in UDMA mode). I'm really not sure that you can make it a whole lot better with the current hardware.

Ideally, you would run a busmastering SCSI RAID card... Sometimes you can run across those cheap at swap meets. Of course, if money grew on trees, a nice new Opteron or D-840 would solve the problem too
 
Old 01-17-2006, 09:10 AM   #3
farslayer
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Or even a nice IDE Hardware RAID controller. 3Ware makes some that are well supported under Linux. a 2 port Parallel IDE RAID controller runs about $120.00 a 4 port card runs about $225.00

When using Software RAID you are making the CPU handle the RAID functions so a high CPU load doesn't surprise me a bit. My goal for servers is to offload that work to the controller.
 
Old 01-17-2006, 09:59 AM   #4
Brian Knoblauch
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After having tried some ATA hardware RAID solutions, I can't recommend those. Haven't had very good experiences. I do plan on trying some SATA RAID later on as the SATA devices out so far seem to be of higher quality than the available "classic" ATA...
 
Old 01-17-2006, 08:07 PM   #5
farslayer
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When looking at Hardware RAID solutions you have to remember that some are "host based" in other words it's a hardware solution that makes use of the system CPU to handle the work load.. I consider them to be mostly junk.. Most host based RAID controllers are designed for windows (big surprise ehh ?) and linu support is spotty at best, and you are still loading the CPU instead of offloading the work to the card as it should be.

if you are going with a hardware RAID solution then it truly needs to be a hardware solution (such as the 3Ware cards) or you may as well just stick with software RAID and avoid the 'host based' headaches.
 
  


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