[nycbug-talk] FreeBSD software RAID?

Steven Kreuzer skreuzer at f2o.org
Thu Mar 8 16:55:23 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.


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