Hi,
On 07 Nov 1998 16:30:29 +0200, Osma Ahvenlampi <[EMAIL PROTECTED]> said:
> Conventional server setup wisdom says to partition /, /usr, /var,
> /home and perhaps /var/spool separately. However, how does the RAID-5
> subsystem perform when multiple md devices are configured to span the
> same physical disks?
In general it's not usually a recommended configuration, but there's no
reason why it cannot be done.
> Obviously, all volumes benefit from redundancy, but let's think about
> the performance side only for now (for that reason, perhaps it would
> actually make more sense to compare striped md's to dedicated disks..)
Often, yes.
> Immediate thought is that number of seeks may increase, resulting in
> more time lost to seek times. On the other hand, sustained bandwidth
> is greater for all volumes. Has anyone run benchmarks to find out
> which is the more powerful factor?
For writes, you will get significantly worse performance, because each
raid-5 write has to span multiple disks (at least the data and parity
disk for that chunk). For reads you have the same tradeoffs as when
deciding whether to place different partitions on the same disk.
> In particular, how does fsck deal with md devices? It parallelizes
> itself for multiple disks, but if the volumes are all actually striped
> over the same disks, fsck will perform better if it's serial.
The "pass" field in /etc/fstab is for exactly this: fsck -a will
serialise devices with different pass numbers. Pass==1 is for root,
pass==2 is for normal devices which fsck knows how to serialise. If you
want to force serialisation on md devices, use larger pass numbers.
--Stephen
