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Re: Problem with compression?
2003-02-24 16:17:43
On Monday, February 24, 2003, at 01:03 PM, John Oliver wrote:
On Fri, Feb 21, 2003 at 09:10:35PM -0500, Jon LaBadie wrote:
On Fri, Feb 21, 2003 at 03:49:32PM -0800, John Oliver wrote:
On Fri, Feb 21, 2003 at 06:14:37PM -0500, Gene Heskett wrote:
Throw in that marketing is usually a bit optimistic in saying its a
20 gigger without compression, and that always needs a fudge factor
when actually estimating, and it likely this will happen.
But fudging by 100%? I don't buy that... :-)
You don't have to. Gene was only talking about a few percent.
No... if my tape is theoretically capable of 20GB uncompressed and 40GB
compressed, and after compression amanda can only fit 20GB on it, that
would hypothetically demonstrate a 10GB un-compressed capacity. Or,
half of what it's actually doing. I do not believe Quantum sells a
10/20 tape drive as a 20/40 I'm sure there *is* some fudging going on,
but not, like I said, 100%. I'm apparently "loosing" about half of the
capacity of my tapes, and I'm puzzled why I'm the only one who sees a
problem with that... :-)
Ummmm.... if you are using server compression, then you (hopefully)
have hardware compression turned off. If not, then you're wasting
space. However, if you have hardware compression turned off, then the
tape itself will store 20Gigs. Even if you have hardware compression
turned on, the likelihood that you will be able to actually get 40Gigs
of data onto it is vanishingly small.
So, you have a tape that holds 20Gigs. Now, Amanda is compressing the
data on the server. So, you have two figures -- the amount of data
that Amanda receives, uncompressed, and the amount that Amanda writes
out to the tape, compressed. Because there is no hardware compression
enabled (and mixing software and hardware compression is _bad_), the
tape drive will only write 20Gigs of data.
The "20Gigs" that you see Amanda writing to tape is the 20Gigs that the
tape will hold. This happens to be already compressed data, but the
tape knows nothing about that.
Now, from what I remember, the problem is that you manage to write
about 9Gigs of (already compressed, uncompressed it was closer to 14-19
gigs -- I'm not looking at the report right now) data out to the tape.
This leaves another 11-odd. The next dump it tried to write to disk
didn't compress at all well -- ie, it was something like 12Gigs of
data, that even _after_ being compressed, was still something like
12Gigs. So, Amanda tries to write out 12Gigs to an 11Gig space, and
fails.
Note that, had it succeeded, the uncompressed data being stored on tape
would have been close to 30Gigs -- the ~12Gigs of the last entry, plus
the uncompressed ~15-19 gigs of the previous entries.
HTH,
Ricky
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