Thanks everyone for the suggestions, they at least give me somewhere to
look, as I was running low on ideas.
More info...
The tape in question have only been used once or twice.
The library is a StorageTek whose SLConsole reports no media (or drive)
errors, though I will look into those linux-based tools.
Our Sun/Oracle service engineer claims that our drives do not require
cleaning tapes. Does that sound legit?
Our throughput is pretty reasonable for our hardware -- we do use disk
staging and get something like 60Mb/s to tape.
Lastly, the tapes that get 200 vs 800 are from the same batch of tapes,
same number of uses, and used by the same pair of SL500 drives. That's
primarily why I wondered if it could be data dependent (or a bacula bug).
thanks!
Stephen
On 09/25/12 02:29, Cejka Rudolf wrote:
>>> We've been using LTO3 tapes with bacula for a few years now. Recently I've
>>> noticed how variable our tape capacity it, ranging from 200-800 Gb.
>>> Is that strictly governed by the compressibility of the actual data being
>>> backed up?
>
> Hello,
> the lower bound 200 GB on 400 GB LTO-3 tapes is not possible due
> to the drive compression, because it always compares, if compressed
> data are shorter that original. In other case, it writes data uncompressed.
> So, in all cases, you should see atleast 400 000 000 000 bytes written
> on all tapes.
>
>>> Or is there some chance that bacula isn't squeezing as much
>>> onto my tapes as I would expect? 200Gb is not very much!
>
> In bacula, look mainly for the reasons, why there is just 200 GB written.
> If the tape is full, think about these:
>
> - Weared tapes. Typical tape service life is written as 200 full cycles.
> However, read
> http://www.xma4govt.co.uk/Libraries/Manufacturer/ultriumwhitepaper_EEE.sflb
> where they experienced problems with some tapes just only after
> 30 cycles! How many cycles could you have with your tapes?
>
> - Do you use disk staging, so that tape writes are done at full speed?
> Do you have a good disk staging? Considering using SSDs for staging
> is very wise. If data rate is lower that 1/3 to 1/2 of native tape
> speed (based on drive vendor, HP or IBM), then drive has to perform
> tape repositions, which means another important excessive drive and
> tape wearing.
>
> My experiences are, that even HW RAID-0 with four 10k disks could not
> be sufficient and when there are data writes and reads in parallel,
> it could not put 80 MB/s to the drive, typically just 50-70 MB/s,
> which is still acceptable for LTO-3, but not good.
>
> Currently, I have 4 x 450 GB SSDs HW RAID-0 with over 1500 GB/s without
> problem running writes and reads in parallel and just after that I hope
> that it is really sufficient for >= LTO-3 staging and putting drives and
> tapes wearing to minimum.
>
> - Dirty heads. You can enforce cleaning cycle, but then return to the
> two points above and other suggestiong, like using some monitoring
> like ltt on Linux (or I have some home made reporting tool using
> camcontrol on FreeBSD), where it would be possible to ensure, that
> your problem are weared tapes, or something else.
>
> Best regards.
>
--
Stephen Thompson Berkeley Seismological Laboratory
step...@seismo.berkeley.edu 215 McCone Hall # 4760
404.538.7077 (phone) University of California, Berkeley
510.643.5811 (fax) Berkeley, CA 94720-4760
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