Brad Campbell wrote:
Callahan, Tom wrote:
It is always wise to build in a spare however, that being said about all
raid levels. In your configuration, if a disk fails in your RAID5, your
array will go down. RAID5 is usually 3+ disks, with a mirror. So you
should
have 3 disks at minimum, and then a 4th as a spare.
/me wonders in the days of reliable RAID-6 why we use RAID-5 + spare?
RAID-6 has saved me twice now from dual drive failures on a 15 disk
array.
It's schweeeeeeeeeett
It's also a lot more overhead... RAID-5 needs to update just one parity
block beyond the data written. As I understand the Q sum in RAID-6, and
watching disk access rates, each write requires the entire stripe to be
read, then P and Q calculated, then written. You can do the P with a
read+write, but since you have to read the entire stripe for Q, you save
a read by recalculating the P from data.
Did I say that right, Neil?
If you are seeing dual drive failures, I suspect your hardware has
problems. We run multiple 3 and 6 TB databases, and over a dozen 1 TB
data caching servers, all using a lot of small fast disk, and I haven't
seen a real dual drive failure in about 8 years.
We did see some cases which looked like dual failures, it turned out to
be a firmware limitation, controller not waiting for the bus to settle
after a real failure, and thinking the next i/o had failed (or similar,
in any case a false fail on the transaction after the real fail). If you
run two PATA drives on the same cable in master/slave, it's at least
possible that this could happen with consumer grade hardware as well.
Just a thought, dual failures are VERY unlikely unless one triggers the
other in some way, like failing the bus or cabinet power supply.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
