Hello,

> blocksize set with mt and in bacula-sd.conf

Unless you are setting "minimum block size" (which you really should
not), Bacula uses the tape drive in variable block size mode, with block
sizes up to the value given in "maximum block size".

Setting a fixed block size with mt (and reading it back with tapeinfo)
is irrelevant.

> btape: btape.c:1082 Test with zero data and bacula block structure.
> btape: btape.c:960 Begin writing 10 files of 21.47 GB with blocks of
> 65536 bytes.

> btape: btape.c:410 Volume bytes=21.47 GB. Write rate = 111.8 MB/s

A 64kb max block size is *way* too small for LTO4. You'll need at least
256kB, possibly 512kB or more to achieve full throughput.

When diagnosing throughput problems, the tests with raw block structure
are more representative of the actual performance of the tape drive,
although the difference will not be that much.

What are you getting in the random data tests? With ~112MB/s for zeroes,
you're still being severely limited by something (most likely block
size). LTO-4 is rated for 120MB/s *to tape*, so you should be aiming for
>110MB/s with random data and 250-300MB/s with zeroes (the latter being
dependent on the compression engine maximum bandwidth).

If you can't achieve that with any maximum block size, your hardware is
probably inadequate for the task.

andrea

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