[nycbug-talk] FreeBSD software RAID?

Miles Nordin carton at Ivy.NET
Thu Mar 8 14:49:55 EST 2007

>>>>> "pw" == Peter Wright <pete at nomadlogic.org> writes:

    pw> i think you may be thinking about your software raid
    pw> configuration data here not metadata...

yeah, I am.  The meaning of the word ``meta'' is very flexible.  The
way I've used it _is_ appropriate---in Solaris SVM for example:

     metadb - create and delete replicas of the metadevice  state

(which is where array geometry and state is stored)

    pw> most decent controllers offer BBU's (Battery Backup Units)
    pw> which not only allow better I/O rates but also help prevent
    pw> loss of data during a catastrophic

well the BBU's plug the RAID5 write hole, so long as they're not
separated from the disks that make up the array, as they would be if
for example the hardware RAID controller card failed.

They are perhaps sold for speeding up databases and mailservers and
(in the old days) NFSv2 servers that do a lot of fsync(), but for the
purposes of this ``i wouldn't touch'' thread, again, it's the RAID5
write hole that I care about, not performance.

The need for them to achieve the illusion of the correct behavior of a
single disk is the reason I think software RAID5 is, AIUI, a bad idea.

    pw> You think vendors like NetApp/EMC/IBM/etc. use software to
    pw> implement low level RAID functionality?

That's a funny statement, but I know what you mean.  

In any case I think we agree on this so far as: my criticisms apply to
RAID-on-a-card only.  The SAN vendors do all have NVRAM that fixes the
RAID5 write hole, but the cards often don't.  Even cards that say ``we
have NVRAM!'' often don't have what the SAN vendors call NVRAM, and
that bugs me a lot because they are basing their business on trying to
confuse people rather than on building trust, which I think is quite
wrong in this space.

    pw> i'd be willing to bet any problems people have had with
    pw> hardware RAID may have been due to misconfiguration of the
    pw> array itself, or a misunderstanding about the fundamentals of
    pw> configuring RAID.

...well...I think there's a misunderstanding about the fundamental
problem of losing your array because you are not able to order the
model of hardware RAID controller that matches your metadata, or not
being able to safely backup this metadata or move it from one card to
another without a lot of hesitant, ominous key-pecks in some clunky
BIOS Blue Screen of Setup.  (RAID metadata, not filesystem metadata)

    pw> snap shotting and RAID/mirroring/etc are two completely
    pw> independent concepts.

we disagree.  I think the idea that you can use the ability to split a
RAID1 to get a very simple snapshot is a relationship between
mirroring and snapshots.  It's also not an invention of mine nor an
odd practice.  In fact it's documented in the EXAMPLES section of the
gmirror man page.
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