[kwlug-disc] MDADM and RAID

John Van Ostrand john at netdirect.ca
Wed Mar 3 20:54:18 EST 2010

----- "Rashkae" <rashkae at tigershaunt.com> wrote:
> Chris Irwin wrote:
> > 
> > 85.4 MB/s in 1m1.380. This is a Hitachi with 16MB cache

85MB/s is close to the low end for SATA disks.

> > 116 MB/s in 0m45.320. This is the Seagate with 32MB cache.

That's the upper limit on typical SATA disks.

Check the manufacturer specs on those drives to see if you're hitting the expectations.

> > Seagates. So let that be a lesson to everybody: Disk cache matters.

It would work better with sequential reads than for random reads.

> Not that much.  There is something very ... strange about those slow
> drives.

The per-disk I/O seems about right. The RAID performance was quite poor though.

The conventional recommendations say to use the same drives within a RAID set. I think the idea is that if you use different drives with different performance metrics (seek-time, rotational delay, transfer speed) you effectively choke performance down to a combination of the worst of each metric, possible making the set perform worse than the slowest drive.

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