From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU]On Behalf Of
Zlatko Krastev
> The requirement for 2 drives is not mandatory, it ought to be just a
> suggestion. LTO can be used as a standalone drive, TSM can use single
> drive or small autoloader with single drive. So it is NOT required but
> recommended.
Yes, technically a single drive is sufficient to do backups. But then I
could use a pair of nail scissors to mow my lawn...
> - single drive reclamation - define reclamation storage pool of type FILE.
> On reclamation remaining data is moved to files and later written to new
> tape volume. Drawback: data is not read when written (sequential
> read+write vs. parallel) thus takes more time. Calculate time budget
> around the clock.
FILE storage pool-based reclamation is dog slow, and expensive of disk
space, particularly if you are backing up database-type data of any size.
I've got a customer trying to do this very thing, and reclamation is
extremely slow.
> - single drive copypools - define following hierarchy DISK -> FILE -> LTO
> (file pool would be also lto reclamation pool). Prevent file->lto
> migration during backups (highmig=100). Perform backups to copypool after
> node backups finish. Allow migration after backup to copypool finishes.
> Drawbacks: filepool must be large enough to hold all backups data. Backups
> should not happen during migration because some object(s) may migrate
> without being copied to the copypool. Again time - data have to be written
> twice through the one-and-only drive. And on the end with one drive there
> is no way to perform copypool reclamation.
Bingo. A single tape drive, because of the lack of reclamation, means no
usable copy pool, no way to use move data to consolidate primary tape
volumes, and no way to use a restore volume command to rebuild bad primary
pool media from copy pool media--in short, a badly crippled TSM backup
system.
> Conclusion: for a small installation data might be not too much, time
> might be enough for all activities (node backups, copypool backup, primary
> pool raclamation, migration, DB backup). Thus neither LTO technology nor
> TSM dictate number of drives to be used but only the business requirements
> you have.
Don't let anyone tell you that a single tape drive is adequate for anything
resembling a real backup system. If you can't afford a real library, you
can't afford a real backup system. My experience with multiple environments
calls for a minimum of three drives--two drives for multitape operations,
and a spare in case one drive breaks down.
--
Mark Stapleton (stapleton AT berbee DOT com)
Mark Stapleton (stapleton AT berbee DOT com)
Certified TSM consultant
Certified AIX system engineer
MCSE
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