Re: Software RAID5 with 2.6.0-test

[mru@users.sourceforge.net (Måns Rullgård)]

Torrey Hoffman <thoffman@arnor.net> writes:

> My experience:
>
> I'm running 2.6.0-test6 on a dual pentium 3 with software raid-5 across
> 5 disks on two different IDE hardware controllers (VIA and Promise). 
> I've got a 224 GB reiserfs partition on that.  
>
> After 8 days uptime, it doesn't seem to have blown up yet.  However I
> don't stress it heavily - just a nightly rsync or two which does a lot
> of reading and writing, and I export my music collection on it via NFS,
> which is a low level of read activity.  

When I tried it, I was running 2.6.0-test4.  The RAID5 was 4 120 GB
Seagate disks on a Highpoint controller.  On top of that, I had LVM,
with ext3 fs.  After just minutes, strange things started happening to
files.  Some had random bits changed in the inode, others were just
trashed.  e2fsck complained a great deal.

I went back to 2.4.21, which is working OK.  A couple of things bother
me, though.  In the dmesg output there are many of these:

raid5: switching cache buffer size, 8192 --> 1024
raid5: switching cache buffer size, 1024 --> 4096
raid5: switching cache buffer size, 4096 --> 1024
raid5: switching cache buffer size, 1024 --> 4096
raid5: switching cache buffer size, 4096 --> 1024

ISTR reading somewhere, that this has a bad impact on performance.

The other thing that I don't like, is the performance of the RAID
array.  The disks individually give ~40 MB/s read speed, but the array
only measures 25 MB/s.  I was of the impression, that RAID5 would give
read speeds at least equal to the underlying disks.  Is this
incorrect?

-- 
Måns Rullgård
mru@users.sf.net