ADSM-L

Re: Using FILE instead of DISK devclass to avoid disk under-utilization

2006-10-26 00:35:51
Subject: Re: Using FILE instead of DISK devclass to avoid disk under-utilization
From: Roger Deschner <rogerd AT UIC DOT EDU>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Wed, 25 Oct 2006 23:34:31 -0500
You probably want to avoid RAID5 for disk storage pools, whether
sequential or random. That can really slow client backups, because RAID5
is quite slow for writing. RAID5 is really only good for read-mostly
applications, so at least you'll migrate quickly. You probably want
RAID10 instead, a striped set of mirrored pairs. (Make sure your RAID10
implementation is NOT a mirrored pair of striped sets, which is quite
unsafe!) RAID10 is a less efficient use of raw disk space, but both
faster and safer than RAID5.

I use disk storage pools mostly, and I have it set up so that I can
easily move space from one to another as demand dictates. It's a manual
process, but it's so easy that I don't mind doing it every couple weeks.
To do this, you need a bunch of smaller LUNs, instead of one huge LUN.

If you do insist on using file devclass, note that you can use multiple
directories in 5.3.

Roger Deschner      University of Illinois at Chicago     rogerd AT uic DOT edu
============= What if there were no rhetorical questions? ==============


>>When I get my new disk in, I plan to make a single RAID5 volume, and make a
>>directory on it for a FILE devclass.  I'll permit something large like 50
>>mounts on that class, and I'll make the volume size somewhere around 250M.
>>
>>I'll define stgpools against this devclass, and I'll control their peak size
>>with MAXSCRATCH directives.  My 2G stgpool will have maxscratch=8, and so on.
>>
>>This will give me most of the speed of my current solution (minus the RAID
>>overhead) and permit the stgpools to grow and shrink as demand varies.