Carlos Mennens wrote:
On Sat, Mar 13, 2010 at 6:50 PM, Bill Davidsen <davidsen@xxxxxxx> wrote:
Having a spare on raid-1 is fairly pointless, it hurts performance and buys
you nothing. Having one more copy of the data built and ready serves you
better.
Can you explain this as I find this interesting. How does having a
/boot partition on 3 drives with 1 spare hurt performance? Are you
saying that I would get better drive performance if I had all 4 disk
partitions active members of my RAID1 /boot? I just don't understand
how the 4th disk doing nothing but acting as a spare would hinder
performance.
In RAID-1 any drive which is a member of the array is already a spare,
they are all the same. So if a drive failed you would just stop using
it. Having a spare makes no sense, since the spare would be rebuilt
using the same data it would have contained as a member of the array.
Note that while the "spare" is being rebuilt the array will be quite
busy and response will be poor, not an issue if /boot is the only thing
on the drives, but more of an issue if the array just uses partitions
and the thrashing of a spare rebuild slows other arrays on the same drives.
Finally, RAID-1 supposedly will read from an unbusy drive if there are
multiple reads against the array, allowing overlapping of seek and
transfer. That's only an issue for /boot if you are trying for a ten
second boot time, but it applies to all RAID-1 arrays.
Secondly, if the above statement also applies to my / partition? Would
you suggest using all 4 drives as active partitions in a RAID5 array
too? If I have a 3 disk RAID5 and one hot spare, do you think I would
get less performance value for my configuration?
No, because the drives are not all the same, so you can't keep a copy of
just the drive ready to fail next. For RAID-[456&10] you want at least
one spare for reliability and fast return to full performance.
Ask again if I didn't give enough detail.
--
Bill Davidsen <davidsen@xxxxxxx>
"We can't solve today's problems by using the same thinking we
used in creating them." - Einstein
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