Fedora14: Strange and intermittent very slow disks on server

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This is a strange one.

I have a home server (Pentium Core 2, Intel ICH10, 1G RAM, 2x SATA 1TByte
disks in Raid1, 1x 2TByte SATA and 1 x 1TByte SATA). This is used for normal
NFS and MythTv usage as well as httpd, network routing openvpn etc. It has been
running for about 2 years and 1 year under Fedora12 with no issues.
I have recently updated it to Fedora14 and updated the disks to the above 
configuration (Was 3 x 320G SATA in Raid5 + 2x 1TByte SATA).
Generally all works fine, but the system has recently started going into a
a running very very slow mode. Once running slow it can take 5mins to login
on an NFS mounted client. Running commands through a ssh take an age. Top 
reports low CPU usage (< 5% but the wait time is above 80%). There is
obviously an issue with disk IO and processes being locked out of disk
access for large periods. Rebooting does not normally clear the issue
but sometimes does.

There do not appear to be any processes doing large amounts of disk IO,
ksysguard reports low disk IO bandwidth in use. I have killed off most of
the processes when the system was running slow with no real effect.
When the problem occurs the Disk I/O is very very slow, normally
"hdparam -t /dev/sd..." gives around 90MBytes/sec. When in slow mode it can
be as low as 2MBytes/sec. Disk I/O tests as the ext4 file system level are
just the same (writes being even worse). This slowness applies to all of the
disks most of which are not being accessed otherwise.
A yum update (on a reasonably fast Internet link) took 6 hours (should
have been more like 10 mins).
There are no messages in /var/log/messages or from dmesg.

It is also intermittent. I have rebooted it a few times, sometimes
when it comes up it is fine, and other times it is not.
The two main RAID1 disks are WD10EARS (Green). I have seen reported some
issues with the performance of these but in my case they appear to work
fine when the system is running ok. Also the system has a WD10EVD disk and
this also goes slow when the problem occurs.

This is with kernel 2.6.35.9-64.fc14.i686.PAE.
I tried installing 2.6.35.6-48.fc14.i686.PAE and booted with that and all was 
fine, however when I went back to 2.6.35.9-64.fc14.i686.PAE it was still fine.
I will run with 2.6.35.6-48.fc14.i686.PAE for a while and see if the problem
re-occurs with that kernel.

Anyone seen this sort of behaviour before ?
Any ideas one where to look ?

Cheers


Terry
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