On Wednesday 26 April 2006 11:31, McGraw, Robert P. wrote:
> For now I do have a holding disk space greater than 300GB that I can use.
> If I set the size greater then 300GB this it should use this space rather
> then shared memory?

I think you are confusing mmap'd memory and shared memory. Shared memory is 
used by taper for something unrelated.

There are three modes that Amanda can use when splitting dumps to tape:
1) Holding-disk mode. The entire dump is written to holding disk, then one 
chunk at a time is written to tape.
2) Split-diskbuffer mode. If your system supports MMAP, and you have 
configured a split_diskbuffer, and the image can't be done in holding-disk 
mode (too big, dumptype does not permit it, etc.), then the holding disk is 
used to buffer one chunk at a time: The chunk is written to the holding disk, 
then flushed to tape, then the next chunk starts, etc.
3) Memory-buffer mode. If there is a problem with split-diskbuffer mode, then 
Amanda will use the fallback_splitsize, and buffer the dump in-memory, one 
chunk at a time.

You were using mode 3 in the example you sent us, because the holding disk was 
not big enough, and your system does not support mmap.

> > When dumping directly to disk, and mmap is not available, then we use the
> > fallback_splitsize (default 10M) instead of the tape_splitsize -- this is
> > because the buffering is done in memory. If you have the memory, feel
> > free to
> > set a larger fallback_splitsize -- it will improve performance a lot.
>
> What does having mmap available do for me?

At present, it is required for chunk-at-a-time buffering of data on the 
holding disk. But that mode is not preferred, because of performance.

> Would this all be a non issue if I keep my holding disk size greater than
> my largest filesystem?

Correct.

Cheers,

--Ian
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