[kwlug-disc] MDADM and RAID

John Van Ostrand john at netdirect.ca
Wed Mar 3 16:22:01 EST 2010

----- "Chris Irwin" <chris at chrisirwin.ca> wrote:
> I'm now doing testing according to John's methodology with some
> different array types. Right now I'm waiting for my RAID10 array to
> sync. But for me, RAID10 shows as RAID10. Unless John manually stacked
> arrays (as you were expecting) and snipped the output.
> md1 : active raid10 sdd2[3] sdc2[2] sdb2[1] sda2[0]
>       975739392 blocks 256K chunks 2 near-copies [4/4] [UUUU]
> Each sync takes about 1.5 hours, so I won't be posting my results
> until later.

The raid10 I created was finished in seconds. I chose not to zero out the device.

What I think my testing showed is that you should get something close to single disk performance with a raid5 configuration. Your single disk was 78MB/s.

Two other factors that might play in this are (and I'm speculating):

Drive cache memory. The SAS drives I'm using are 10K rpm scsi-class sata disks which are faster than typical consumer sata disks. This may allow the disk to queue more data for writing so it can efficiently write more data at once making it more efficient.

Command Queuing. Depending on the SATA chipset and driver used it may not support command queuing meaning the disk may be writing the blocks as they come in rather than returning control. With Sata this is called NCQ (native command queuing) here is a doc that describes it a little more: http://forums.serverbeach.com/showthread.php?t=7333 . My CentOS 5.4 box has a queue depth of 32, my Fedora 11 box has a queue depth of 1.

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