On Thu, 18 Dec 2008, Jesper Krogh wrote:

> > 100-200Gb ram and systems capable of addressing that amount of memory are
> > still far more expensive than a stack of flash drives, else I'd use them.
>
> But do you need to spool a complete tape? In order to avoid doing "evil" stuff
> to you tape drive, much less is sufficient.

I'm multiplexing anything up to 20 jobs at a time. To ensure that small
incremental and diff jobs are dumped in one hit and to ensure that full
backups are laid in as large chunks as possible, this is the kindof size
which is required.

> > My concern isn't just backup run time.
>
> So you'd like to spool a complete Job? Whats you average job-size? (mine is
> less than 8GB)

Full backups run 500Gb to 1Tb apiece, the average nightly incremental is
about 80-150Gb - multiplied out by 90 filesystems.

You get the idea.

> Concerned about job run time, its my impression that spool space only speeds
> up incremental/differential.

It does at the moment. HOWEVER if spooling isn't used then jobs are
interleaved on the tape at record time, resulting in massive shoeshining
on restores and restore throughput rates measured in kb/Sec instead of
25-40Mb/sec

> Whats the time consuming part in this? Seeking on tapes? Neither SSD's or
> memory will change that. AFAIK the spooling area is only used when going TO
> tape, not FROM tape.

Using large spool areas increases the size of the chunks dropped to tape
and thus allows the tapes to stream for longer periods. ONE of my backups
might span 5 LTO2s

> Can you give some numbers, so we have a feeling about the sizes you talk
> about?

250Tb startup, 1Pb within 18 months, and growing past that. Filesizes
measured in tens of Mb apiece.




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