Re: email server - how to
On Wed, 30 Jun 2004 21:23, Dave Watkins <debianisp@snorks.dyndns.org> wrote:
> Andreas John wrote:
> >> Best to use 2U machines with the maximum number of disks IMHO. A 2U
> >> machine should be able to have 5 disks.
> >
> > I say: 9 Disks without problems. e.g. pcicase
> > http://www.pcicase.de/catalog/produktweb/IPC-C2-X/IPC-C2D.htm
>
> The question is with that many disks is a single raid 5 going to be
> enough redundancy... Thats an awful lot of data to loose if 2 drives
> fail. May be worth thinking about RAID6 or a couple of RAID5 arrays striped
If you have two RAID-5 arrays striped then two disks can fail and lose all
your data. If you have a 10 disk setup where one disk has already failed,
and if all disks are equally likely to fail, then on a single RAID-5 any disk
failure will lose your data while on a pair of striped RAID-5's the chance
will be 4/9 that the next failure will lose the data.
However in a RAID-5 when one disk has failed there is more work for the
remaining disk, so it may be more likely that the RAID-5 which has already
lost a disk will lose a second than having a disk die in a RAID-5 that's
working fine.
Another issue is that physical issues (vibration and temperature) can cause or
trigger disk death. As a RAID-5 is likely to be comprised of disks that are
near each other there may be a pattern to disk death.
I would hope that RAID-6 would be significantly more reliable than RAID-5.
However there are lots of other causes of data loss. If reads don't occur on
all disks at the same time with checking of both parity blocks then a RAID-6
system will still fail if a disk returns bad data and claims it to be good.
Performance will be better if you don't have to read all blocks in each
stripe for every read, so I expect that most systems will support turning off
the feature to read the entire stripe (and it may be the default for some).
There are lots of physical issues that can take out multiple disks, anything
that can take out two disks can probably take out three just as easily.
These physical issues include repairmen who use a hammer as a CPU
installation tool (this is not a joke).
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