Hi Jon,
Am 14.04.25 um 19:36 schrieb Jon Schewe:
I have not set the Maximum File Size for my tape devices as I assumed
this was only for file based backups. I now see https://docs.bareos.org/
bareos-19.2/TasksAndConcepts/AutochangerSupport.html#tapespeed-and-
blocksizes points to changing this value. Is there a recommendation for
LTO-8 and LTO-9 devices?
My personal recommendation for newer LTO generations (i.e. LTO-6 and
newer) is to set Maximum File Size to 250 GB.
Just to add a bit of background information:
As you already know tapes basically contain a stream of blocks. However,
historically (i.e. when using tar/cpio directly) you would put multiple
files onto a tape. Therefore end-of-file marks are placed on the tape
and the drives can fast-forward to these.
Bareos makes use of this by putting end-of-file marks onto the tape at
least every Maximum File Size bytes.
When putting such an end-of-file mark into the tape, there is also a
guarantee that the data including the mark was actually persisted to the
tape (and is not just in the drive's buffer).
The downside of this is, of course, that the tape buffer is flushed
every time a mark is written. As modern drives have pretty large caches
(1 GB and maybe even more), the drive will never come up to speed as
you're basically synchronously flushing the buffer every time before it
gets full.
It looks like changing the Maximum Block Size can be a challenge to keep
compatibility with older backups. Does changing this make a large
difference in performance and make it worth the risks?
Please do not mix up Maximum File Size and Maximum Block Size. These are
totally different things.
Maximum Block Size nowadays defaults to 1 MB. Outside of testing there
should be no need to ever touch this parameter.
Read compatibility was also added to Bareos 23: When the SD encounters a
block that is larger than its configured Maximum Block Size, it will
automatically retry with 1,2,4,8 and 16 MB block size.
So while the warnings in the whitepaper about changing the Maximum Block
Size are still valid, restore should still work even with a
misconfigured SD.
Best Regards,
Andreas
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