Linux Software RAID5 Performance

[Velocet]

On Wed, Apr 03, 2002 at 02:49:00PM -0500, Michael Prinkey's all...
> Indeed, the multiple processes accessing the device made significantly 
> degrade performance.  Fortunately for us, as well, access speed is limited 
> by the NFS/SMB and the network, not by array performance.  Unfortunately, 
> the unit is online now and I can't fiddle around with the settings and test 
> it further.
> 
> WRT reliability, we have seen the array drop to degraded mode because of a 
> single drive failure.  We have also a single drive take down the entire IDE 
> port.  This results in the md device disappearing until you swap out the 
> offending drive and restart the array.  There is no data here.  Usually one 
> drive goes and the array goes into degraded mode and starts reconstructing 
> on the spare.  Then the second goes and the array disappears.  It is a bit 
> disconcerting to do ls /raid and get nothing back.  Changing out the drive 
> and restarting pulls everything back.
> 
> I can honestly say that the only data loss that I have had on these arrays 
> came when a maintenance person completely unplugged one of the arrays from 
> the UPS.  It caused low-level corruption on 5 of the 9 drives in the array.  
> We ended up using a Windows 98 boot floppy with Maxtor's Powermax utility to 
> patch them all back up.  It took many hours.  This is the WORST possible 
> scenario, BTW.  Even reseting the system gives the EIDE devices a chance to 
> flush their caches and maintain low-level integrity.  Cutting the power can 
> leave the array/drives inconsistent on the filesystem, device (/dev/md0), 
> and hardware-format datagram levels.  So, lock your arrays in a cabinet!  8)

ok get an EIDE RAID controller with battery backed-up ram onboard. We pulled a
bunch of the SCSI equiv of such from a netfinity server a customer pawned off
on us.  Rather nice. (anyone want to buy? :)

/kc

> 
> Mike
> 
> >From: Jurgen Botz <jurgen at botz.org>
> >To: mprinkey at aeolusresearch.com (Michael Prinkey)
> >CC: beowulf at beowulf.org
> >Subject: Re: Linux Software RAID5 Performance
> >Date: Wed, 03 Apr 2002 10:25:31 -0800
> >
> >Michael Prinkey wrote:
> > > Again, performance (see below) is remarkably good, especially 
> >considering
> > > all of the strikes against this configuration:  EIDE instead of SCSI, 
> >UDMA66
> > > instead of 100/133, 5400-RPM instead of 7200-RPM, and master/slave 
> >drives on
> > > each port instead of a single drive per port.
> >
> >With regard to the master/slave config... I note that your performance
> >test is a single reader/writer... in this config with RAID5 I would
> >expect the performance to be quite good even with 2 drives per IDE
> >controller.  But if you have several processes doing disk I/O
> >simultaneously you should see a rather more precipitous drop in
> >performance than you would with a single drive per IDE controller.
> >I'm working on testing a very similar config right now and that's
> >one of my findings (which I had expected) but our application for this
> >is not very performance sensitive so it's not a big deal.
> >
> >A more important issue for me is reliability, and I'm somewhat
> >concerned about failure modes.  For example, can an IDE drive fail
> >in such a way that if will disable the controller or the other
> >drive on the same controller?  If so, that would seriously limit
> >the usefulness of RAID5 in this config.  In general how good is
> >Linux software RAID's failure handling?  Etc.
> >
> >:j
> >
> >
> >--
> >Jürgen Botz                       | While differing widely in the various
> >jurgen at botz.org                   | little bits we know, in our infinite
> >                                   | ignorance we are all equal. -Karl 
> >Popper
> >
> >
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-- 
Ken Chase, math at velocet.ca  *  Velocet Communications Inc.  *  Toronto, CANADA