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Rumko wrote:
> Item n: Spooling data while despooling it
> Origin: Rumko <[EMAIL PROTECTED]>
> Date: 21 June 2006
> Status:
>
> What: Currently when fd runs out of data or maximum spool size is
> reached, the sd starts despooling. While it is despooling, the fd does
> not spool the data until all data has been despooled. For performance
> reasons (and it would prolong tape's lifetime), the fd should spool
> data while the data is being despooled by the sd.
>
> Why: Better performance on tape drives (writing would be
> prolonged, because while writing the data, more would be spooled and
> made available to the sd) and longer lifetimes for the tapes.\
Both could be argued: Is performance really better when your backups
system is reading from disk and writing to it simultaneously? Probably
not. And given the performance hit you take when during those
simultaneous reads/writes, your backup system might actually start
thrashing and not keep up with the data rate required to feed your tape
drive. The result of that would be the dreaded shoe-shining, the exact
thing that data spooling is supposed to cure. (I'm pretty sure my backup
system will not be able to keep up with spooling/despooling as it is
unfortunately has quite slow storage that can barely keep up with
despooling to the drive alone.)
If one were to implement this, it should be made optional and probably
fine tunable concerning the acceptable data rates (ie settings like:
"stop spooling if read rate goes below XXXX" or "limit spool rate while
despooling to XXXX" ...).
Greetings,
Michel
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