Dear Adam,
In message <20190207113145.144af...@teln.shikadi.net> you wrote:
> > Spooling can reduce overall throughput because the data is
> > sequentially written to disk and then read back.
>
> This is what got me. I thought it was a buffer to ride out variations
> in disk read speed (like the mbuffer program) but it's not. The
> purpose is to get data off clients as fast as possible for later
> writing to slow tape.
I have a different understanding of the function. there is no need
to read "data off clients as fast as possible" - if your clients are
fast, they have no problems to provide the data at lower rates,
too. It's the other way round; if your clients are slow so they
cannot feed the data fast enough to keep the tape streaming. This
often happens when you run for example incremental backups over a
big data set (say, millions of files) with only little changes.
In this case the client gets the time it needs to traverse the fole
directory tree, and when done, you have all date to be backed up
collected in the spool area which is then fast enough to kepp the
tape happily streaming.
> Consequently spooling works best when the spool file is large enough to
> contain one whole tape's worth of data, and you have enough clients
> backing up that there is always a complete spool file ready to write
> out to tape.
This is not necessary. Or only one possible special case.
> Anything less than this and spooling will slow things down.
This is not correct, if you consider incremental and differential
backups.
Best regards,
Wolfgang Denk
--
DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: w...@denx.de
If programming was easy, they wouldn't need something as complicated
as a human being to do it, now would they?
- L. Wall & R. L. Schwartz, _Programming Perl_
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