[NCLUG] RAID-1 and disk speed

Sean Reifschneider jafo at tummy.com
Tue Oct 20 14:26:50 MDT 2009


On 10/20/2009 10:23 AM, grant at amadensor.com wrote:
> I have a question.   Has anyone tried pairing very different speed disks
> with a mirrored RAID array?   Do you manage to get the faster one to

See the man page for "mdadm":

   -W, --write-mostly
        subsequent  devices listed in a --build, --create, or --add com-
        mand will be flagged as 'write-mostly'.  This is valid for RAID1
        only  and  means  that  the  'md' driver will avoid reading from
        these devices if at all possible.  This can be useful if mirror-
        ing over a slow link.

> My thought is that if I were to pair a very fast SSD with a more reliable
> traditional magnetic disk, could the application see at least some of the
> benefit of the improved performance, with the added security of the real
> disk?

I'd expect that if you had an SSD paired with a spinning disc that it would
mostly read from the SSD because the queue would be smaller on the SSD.

However, I will say that I'm really not that happy with the performance of
my Intel 80GB SSD, which does have the "fixed" firmware.  Reads are very
fast, opening OpenOffice the first time is quicker than with a spinning
disc.  But, I'm running into stalling now fairly reguarly, which just hangs
the system for around 15 to 30 seconds.  When I update the tags on a bunch
of photos or the like, the system will just hang periodically.

So, until trim support is in there and I can get a bigger pool of
ready-to-write cells, I'm not sure the performance is there for a load that
includes much writing.

Reading is very fast.

Sean
-- 
Sean Reifschneider, Member of Technical Staff <jafo at tummy.com>
tummy.com, ltd. - Linux Consulting since 1995: Ask me about High Availability

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 252 bytes
Desc: OpenPGP digital signature
URL: <http://lists.nclug.org/pipermail/nclug/attachments/20091020/352f5b9c/attachment.pgp>


More information about the NCLUG mailing list