RAID controller question...

[Caskey L. Dickson]

On Mon, 7 Sep 1998, Eric Billings wrote:

> > which I think will work, but can't find pricing on either and am not
> > sure if LINUX drivers exist for each.  The controllers are:
> > 
> > 	AMI MegaRAID® Ultra2/5 (Series 438)
> > 
>
> We are currently running the AMI MegaRAID UltraGT under Linux 2.0.31 with
> a vendor-supplied driver.  It is configured as RAID-5 with 8 18GB drives
> spread across the 3 channels of the controller.  It has 128MB on-board
> cache.  Performance with this is fine - not great.
> 
> For burst writes and reads we are measuring 20-30MB/s for files < 50K that
> are cacheable.  Sustained writes to disk are about 2.2MB/s with 8 disks in
> a single logical volume and data-striping enabled for files > 1GB.
> 
> We are currently testing data recovery.  This takes a while because
> loading the disk with >100GB of data takes more than 12 hours.
> 
> CAVEAT EMPTOR: Hardware supported RAID requires that you can get to the 
> controller's code to rebuild a lost disk.  This means that you must take 
> the system down and reboot to get to the PCI controller during the boot
> sequence.  This eliminates advantages such as hot-swap and keeping the 
> host node accessible during the rebuild process.  At least so far...
> 

I have a little expierience with RAID units and have found the most
enjoyable(?!?) ones to be RAID enclosures with onboard controllers.  They
present a single SCSI device to the bus and use an internal chain for the
drives.  Most have small status displays that one can use to control the
unit, we used a VT100 terminal.  With these enclosures and Hot
swappability I've had no trouble with
failover and recovery on a live database.  Depending upon your
manufacturer you could experience quite a few drive failures, however with
some care and feeding 24x7 uptime was almost attainable. 

I have never been a fan of on board RAID controllers and probably never
will be. Perhaps it is my affinity for encapsulation and separation of
functionality.

The configuration we had most success with was using a standard disk for
the OS/Boot and mounting the RAID unit as supplimentary storage.

OfC your decisions are based upon what you perceive your RAID unit
protecting you against.  For us, it was data corruption and downtime due
to drive failures.

C=)

P.S.  Never, never, never accidentally unplug the RAID unit from the UPS,
that makes *everybody* unhappy. :-)

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Caskey <caskey*technocage.com>       ///                pager.818.698.2306
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