resync performance drop after I/O in RAID 5

resync performance drop after I/O in RAID 5

[Joel Lilienkamp]

We are observing a strange performance drop on resync after an I/O is
performed to a RAID5 device.  This is using the RAID5 / md support in
Linux 2.4.20.  It is observable on at least two different disk
controllers, so it doesn't appear to be a specific hardware limitation.

To reproduce:

(1) Create a RAID5 device, say /dev/md5, from four partitions on
different disks, chunk-size = 256.
(2) Observe resync performance vi 'cat /proc/mdstat'.  We are seeing
around 30MB/Sec on one controller, and 50+MB/sec on another.
(3) Do a small read from the RAID device, such as 'dd if=/dev/md5
of=/dev/null bs=1024 count=1'.
(4) Re-observe resync performance vi 'cat /proc/mdstat'.  After the I/O,
we are seeing resync drop by about 60%, to 11-12MB/Sec on one
controller, and 15MB/sec on the other.

The performance never recovers to the previous higher level, short of a
reboot (which starts resync from the beginning) or recontructing the
device.  I/O to the individual sub-partitions does not impact this.  We
do not observe similar issues with RAID1.

I ahve scoured the various lists and postings to find anything about
this, but I haven't located anything.

Thanks very much for any help that can be offered.

Joel Lilienkamp