Thanks for the elaborate reply. Just a few more querious questions.

Alan Brown wrote:
> 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.

Are you having that many clients or is it actually larger volumes that 
have been split up in smaller filesets?

>>> 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.

Absolutely.

>> 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

Yes, I use spooling even for our +2TB volumes.

>> 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

I can have the same for LTO3, but that due to a few large jobs running 
full backups concurrenly.

>> 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.

Still over Gigabit netwok or do you have better infrastructure?

-- 
Jesper

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