If one does a realistic examination of restore times, tape mounts, unless
you have very slow tapes, usually won't come into play.

Let's start the discussion with service level agreements.  For instance,
all of us should have in place agreements with our customers that sound
something like this:

1. For single file restores, we offer a restore time of one hour or less.
2. For a directory tree restore up to 200 MB, we offer a restore time of
one hour or less.
3. For a full disk restore up to 4 GB, we offer a restore time of four
hours or less.
4. For a full system restore up to 100 GB, we offer a restore time of 24
hours.

Wiggle these to meet your needs.

Think about what many of us can offer now.  Are the numbers above
reasonable from a business standpoint?  Most of us state we'll do it as
fast as possible when we should be putting in place SLAs that more
accurately reflect reality.

Restore statistics:

99% of restores are for single files, or small numbers of files from
yesterday.  Not a problem with ADSM.
1% of the restores are bigger than this: complete directories, all the data
for an application, a complete disk, a complete system.  This flat does not
happen very often, but one should be concerned (but not overly so) about
it.

Let's look at the larger restores.  Pick a hard one like the full disk
restore.  Assume a 30 day data retention (verdeleted, verexists, retextra,
retonly all set to 30).  Worst case, 30 tape mounts will be required during
a full restore of the disk.  Most of the data will be on the first tape
with small amounts scattered on the rest of the 30.  In fact, looking at
typical usage, the same files tend to change so the same files will show up
on many of the tapes.  So during a restore, these files will only be
restored once, and perhaps from the most recent tape.  Tape mounts just
went way down.

But let's keep it at 30 mounts.  Say each mount takes two minutes and the
time to reach data is another minute for a total of three minutes per tape.
 Then we can do some real work and move data.  For this case, we'll expend
90 minutes of our four hour window.  Are we in trouble?  I don't think so.
 We should meet our SLA for this restore.  Will we actually need to mount
30 tapes?  Probably not, since we've got reclamation going on as well.

If tape mounts really become a problem, and I'm maintaining this is the
least of your worries about large restores, one can always use collocation
to reduce them.

Think about your data.  It's usage and its change patterns.  Think about
your other protection mechanisms: RAID, shadowing, etc.  Think about your
restore requirements and SLAs.  For those of you still believing GFS
(Grandfather, father, son) backups are still better, think about this: you
have to mount all of the incrementals between a full even though most of
same files change everyday.  Using this method, more files are actually
restored and then deleted than ADSM would actually restore!

BTW, one should test the restore times before publishing SLAs.  However, if
one applies reason and logic, this just isn't all that hard.

Thanks,

Kelly J. Lipp
Storage Solutions Specialists, Inc.
www.storsol.com
lipp AT storsol DOT com
(719)531-5926