ADSM-L

Re: NetWare, Compression & Slow Backups

2002-04-10 15:51:50
Subject: Re: NetWare, Compression & Slow Backups
From: "Wayne T. Smith" <ADSM AT MAINE DOT EDU>
Date: Wed, 10 Apr 2002 15:50:11 -0400
On 10 Apr 2002 at 16:18, Daniel Sparrman wrote, in part:
> You shouldn't use Compressalways YES. If the file grows, instead of
> shrinks, TSM will keep on compressing the file. This will make the
> backups take a whole lot longer.

I disagree with Daniel's conclusion ... at least for some cases.

*If* you have a case where compress=on and most files benefit from
compression, then I think CompressAlways=Yes can be very beneficial.
Yes, the file sizes of some files will grow slightly, but this is far
superior to having *SM discarding the entire transaction group and
resending everything ... especially with large transactions that take
considerable time to transmit.  You lose a little instead of losing a
lot.

On the other hand, I've found (no scientific study!) compression rates
relatively independent from backup times.

My site has gone from mostly compressing to mostly not compressing. The
effect has been less traumatic effects at the client and somewhat
larger disk pool needs, as very compressible, but not compressed, data
takes substantially more disk space at the backup server.

My short answer:

(1) Use Compress and CompressAlways together.
(2) Mildly avoid compress.  If you compress, do because you need to,
not because compression values say you're transmitted 1/2 the # bits.

Your mileage may vary, wayne   :-)

Wayne T. Smith                          ADSM AT Maine DOT edu
ADSM Technical Coordinator - UNET       University of Maine System
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