Ben,
On Thu, 16 Mar 1995 17:55:50 -0600 Ben William Brumfield said:
>1) What kind of increase in tape mounts per nightly backup job are we
>looking at?
We never ran without collocation, so I don't know what the change will be
for you. However, I have a suggestion that I think makes sense. We use
it, but I've never done a careful analysis or gotten any feedback from
IBM on whether it makes sense or not. The suggestion is to make sure
you don't set your LOW migration threshold too low when you turn
collocation on. If you do, I think this will cause increasingly extra
tape mounts. The lower you set LOW, the more tape mounts you will get
(not linearly more, but exponentially more). The reason for this, (this
is my theory) is that when a migration process runs, ADSM selects
filespaces which have large amounts of data on the disk storage pool
first. As the migration process proceeds, smaller filespaces are
selected for migration. Each filespace could potentially be migrated
to a different tape, and therefore require a tape mount. By keeping
the LOW threshold high (we use 30%), you avoid migrating all those
smaller filespaces.
If anyone from ADSM Server Development sees this, I would love to get some
feedback on whether my theory holds any water or not.
>2) We already have several hundred 3480 tapes worth of saved user data, many
>of which are nearly half empty. I understand from the documentation that
>enabling collocation will essentially add at least an extra tape for each
>node to our usage.
I don't see why you will need an extra tape for each node. What
documentation are you referring to? Even with collocation, data from
multiple nodes may be placed on the same tape. Collocation tries to reduce
the number of tapes that a node has data on when migrating data or
reclaiming tapes. It does so by selecting tapes which already have some
data from that node on them. If you want to reduce the number of tapes
that a node has data on to a minimum, THEN you will want to allocate
additional tapes. The more tapes that are available, the fewer nodes there
will be on each tape. But it's not required to have more tapes.
..Paul
Paul Zarnowski Phone: 607/255-4757
Cornell Information Technologies Fax: 607/255-6523
Cornell University US Mail: 315 CCC, Ithaca, NY 14853-2601
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