[nycbug-talk] FreeBSD software RAID?

Peter Wright pete at nomadlogic.org
Thu Mar 8 17:21:23 EST 2007

> On Mar 8, 2007, at 4:33 PM, Peter Wright wrote:
>> hmm...i guess i'm just not sure what you mean by "RAID-on-a-card".
>> i'll
>> also have to look into the "RAID5 write hole" as that's a new term
>> for me
>> as well.
> When data in a raid stripe is updated, the parity also needs to be
> updated so that an XOR on all of the disks will be zero. (That what
> allows you to rebuild the disk when they fail). The write hole is is
> because you can't update two or more disks atomically so the stripe
> can become damaged during a crash or power outage if data is written
> to disk, but the parity bits didn't get calculated yet.
> If this occurs, when the set is being rebuilt, the parity will be
> inconsistent and the reconstruction will result in garbage data and
> it will have no idea it is generating garbage data. (RAID-1 also
> suffers from this)
> However, this is nothing to concern yourself with if you have a RAID
> controller with a battery backed cache.
> If you are going with a software RAID-1/RAID-5, stick the machine on
> a UPS and make sure it has enough juice to shut down the machine and
> have write caching turned on in the drives.

ahh..i'm familiar with that issue, never thought to give it a name.



Peter Wright
pete at nomadlogic.org

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