Hi there,
(The reported problem has been noticed on several machines, with different
hardware, and different 2.6.x kernels, up to 2.6.12)
I use EVMS with MD-RAID based storage, usually buiding several RAID-sets out
of partitions from the same physical disks, for example (just an example):
1 MD RAID-1 set for a /boot made with 2 partitions out of 2 disks
1 MD RAID-1 set for a / (root fs) made with 2 partitions out of the 2 same
disks
1 MD RAID-1 set for a swap made with 2 partitions out of the 2 same disks
1 MD RAID-1 set for a LVM containg different other volumes made with 2
partitions out of the 2 same disks
Using EVMS, the RAID-sets are actually built from dm-managed devices.
Using a 2.4 series kernel, I noticed the following :
---------------------------------------------------------------
1/ When starting RAID-sets, MD usually issued a warning such as:
<< md1: WARNING: [dev fe:01] appears to be on the same physical disk as [dev
fe:00]. True protection against single-disk failure might be compromised. >>
The dm devices were actually NOT on the same physical disk, so this warning
was of no consequences.
2/ When RAID-sets went ouf of sync and had to resync, MD resynced them one at
a time, each in turn, which was a correct behaviour.
Using a 2.6 series kernel, I noticed the following :
---------------------------------------------------------------
1/ When starting RAID-sets, MD doesn't complain about devices "being on the
same physical disks" anymore.
2/ When RAID-sets get ouf of sync and have to resync, MD now resyncs them all
in parallel, at the same time, which results in resyncing several RAID-sets
made from the same disks at the same time.
=> This seems to slow down resyncing very much for all RAIDs, and also
probably causes huge disks arms/heads movements (my ears tell me about
this ;-)
Regarding resync speed, even when one single RAID-array is resyncing :
With 2.4 kernel : With a mostly idle system, RAID resync speed usually goes as
high as /proc/sys/dev/raid/speed_limit_max allows, unless other system
activity slows it down.
With 2.6 kernel : Even with a mostly idle system, RAID resync speed seems to
always stick to /proc/sys/dev/raid/speed_limit_min, and never goes much
higher.
So it seems that the behaviour of the 2.4 MD regarding this is much closer to
what is desired, compared to the behaviour of the 2.6 kernels. (Comparison
done on the same system with the same overall configuration and activity).
Any idea about how this could be improved ?
(Please copy me on answers, as I'm not subscribed to the linux-kernel ML)
Cheers.
--
Michel Bouissou <[email protected]> OpenPGP ID 0xDDE8AC6E
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