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Re: RAID Suggestion for webserver

> > Okay, as you said, with RAID10 and 4 40G HDs, usable space is 80Gs.
> >
> > On the other hand, with RAID5 and 3 40G HDs, usable space is also
> > with 1 spare HD for rebuilding.
> >
> > The question becomes... which provides more performance and is more
> > reliable?
> RAID10 will give you the most performance. Not only do you have 4 disks
> working for you all the time, instead of 3 with RAID5, you (or more
> accurate: your CPU) also don't have to calculate the parity which is
> used by RAID5.

The CPU won't be handling this... the 3ware RAID card (hardware) will
perform the parity calculations, so RAID 5 won't cause that type of
slowdown due to additional CPU utilization.

> Both will survive a 1 disk crash with no problems and both will appear
> as a RAID0 array when running in degraded mode. However, the reliability
> is different when a second disk fails. In RAID5 with spare you are out
> of luck when a second disk fails while the spare is rebuilding.
> With RAID10 and 1 failed disk, you only have the disk that is in the
> same stripe as the failed disk that is save to fail.
> I'm not sure if the raid card supports a stripe of two mirrors. This
> setup will survive a 2 disk failure.

With the RAID5 with 3 disks and 1 spare, the only time the array would be
vunerable would be during reconstruction onto the spare disk. Once that is
done, the array will be fully restored, and could survive another failure
in any disk.

As you mentioned, the RAID10 with 4 disks could also survive 2 failures,
however (again as you mentioned) the 2nd disk that failures cannot be part
of the same stripe as the original failed disk.

Performance while the RAID5 array is degraded won't be too bad due to the
fact this is hardware RAID and not software RAID, and the hardware's
dedicated RAID chips will handle the computations.

It seems RAID5 would be a safer solution as long as another failure does
not occur during the reconstruction onto the spare. Hum... how long would
the reconstruction take for a 40G hd? I'm guessing 30-40 minutes? Would
that be about right?

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