Software RAID issues (was Re: Suggestions solicited, server bring up)

Ben Scott dragonhawk at gmail.com
Tue Nov 24 14:34:32 EST 2009


On Mon, Nov 23, 2009 at 1:09 PM, Tom Buskey <tom at buskey.name> wrote:
>> http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5_performance:
>> "... known as the write hole ..."
>
> Thanks for clarifying that for me.

  For the record, there doesn't seem to be any universal agreement as
to what "RAID 5 write hole" actually means.

> I'm corrected.  RAID 5 will always have to do parity and RAID 0 does not.

  I find it's a good idea to keep the definitions in mind when talking
about this stuff.  The numbered levels tend to confuse things.  In
discussions, I usually re-state the definitions regularly for this
reason.

RAID 0 = striping without redundancy
RAID 1 = mirroring
RAID 5 = stripping with distributed parity
RAID 6 = stripping with double distributed parity
RAID 10 = striped mirrors

  I've left out the RAID levels nobody uses, like 3 and 0+1, and the
higher-numbered levels which are ill-defined.

> I once replaced my 120 GB drives with 500 GB drives to increase the pool.
> It didn't seem slow to me, but..

  A big part of rebuild performance is the load on the system.  If the
system has a light I/O load, there's enough spare capacity that
rebuild overhead won't be much noticed.

  The same goes for performance of software vs hardware RAID.  Most
people run their I/O benchmarks with zero load, because they want
controlled conditions.  However, software RAID robs resources from the
host, so load matters.  Many systems have resources to spare, so this
is okay.  But on a heavily loaded system, this may matter.  Say you
we're talking members of a database server cluster backing a busy web
site.  CPU, RAM, and I/O will all be in demand.  You may not want to
give up resources to that.

  OTOH, it may be cheaper to buy another cluster member than upgrade
your storage controller.  But OTOOH, before adding another server,
don't forget to consider impact on rack space budget, power budget,
heat budget, and the environment.  "You can't do just one thing."

-- Ben



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