[mythtv-users] LVM RAID 5 partition schemes

[Greg]

> 
> > Raid 5 runs quite a bit slower when its degraded (missing drives).
> 
> I hadn't heard this.  Any idea why this is the case?  I would 
> understand that you don't get the speed boost during reading 
> from the extra drive, but I would have guessed that it would 
> give higher read throughput than a single drive.
> 
> Dan
> 

I believe it's because the data from the missing drive has to be recreated
from the parity data on the other drives in the array and this reduces the
performance significantly.

From:
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-
us/sag_diskconcepts_18.mspx

"RAID-5 volumes have better read performance than mirrored volumes When a
member is missing, however, such as when a disk has failed, the read
performance is degraded by the need to recover the data with the parity
information. Nevertheless, this strategy is recommended over mirrored
volumes for programs that require redundancy and are primarily
read-oriented. Write performance is reduced by the parity calculation. A
write operation also requires three times more memory than a read operation
during normal operation. Moreover, when a volume fails, reading requires at
least three times more memory than before the failure. Both conditions are
caused by the parity calculation."


Greg