Hi,
On 5/20/2006 9:45 PM, Robert Nelson wrote:
It is possible to query this information from SCSI tape drives using the LOG
SENSE command, page 0x31.
Hmm. According to the documents I read, LOG SENSE itself is optional,
and page 0x31 is vendor specific (according to SPC-3 draft, and SSC-3
draft, and assuming I understood the text correctly). Instead, some data
is available in log page 0x0c, which is mandatory, but there they only
talk about approximate values.
Only seeing what the standard calls "native capacity" of a tape makes
one aware of the problems when trying to determine tape capacity:
"3.1.43 native capacity: The capacity assuming one-to-one compression
(e.g., compression disabled), the medium is in good condition, and that
the device recommended typical block size is used."
Anyway, in typical scenarios, the only reliable way way of guessing tape
capacity (or remaining capacity) is estimating based on individual data,
i.e. using existing tapes with similar data on them as reference.
Arno
It contains the remaining capacity and total maximum capacity in kilobytes.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Timo Neuvonen
Sent: Saturday, May 20, 2006 6:43 AM
To: bacula-users@lists.sourceforge.net
Subject: [Bacula-users] Re: How much available space on the tape?
That's not how hardware compression for tapes work. You write
blocks of, say, 1 GB to to tape, and depending on how good that
data can be compressed the length of tape (usually, the length
of the track(s)) used vary. You really can't know how much
data fits on a tape before measuring it, and if hardware
compression is on your results areonly valid for your test data.
So, one solution would be using of software compression? I guess Bacula
allows it?
Then, the amount of data written to the tape (drive) would be known exactly,
and this would propably make it possible to know exactly how much tape there
is left.
Anyway, for rough estimates one propably could trust the hardware
compression to work with almost constant factor too, as long as the
environment (including the type of data) remains constant. Information on
the amount of data written to tape drive is available in the logs, but it's
beyond my knowledge how it possibly could be used to automate the recycling
decision.
Actually, it would be nice to be able to define "mark volume full at the end
of job if xxx gigabytes has been appended". This could propably be done with
an external script, that was called after the finished backup? However, I
find calling external scripts to manipulate the database more error-prone
compared to built-in features. Am I just too paranoid?
If one tape could normally store eg. 5-6 backups, this actually has little
practical value -a constant number of backups before recycling is propably
enough, considering the required tolerance. But if a single tape could
handle eg. 25-30 backups, this could become more valuable -it might allow
eg. one extra month to be available before recycling.
--
TiN
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--
IT-Service Lehmann [EMAIL PROTECTED]
Arno Lehmann http://www.its-lehmann.de
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