Re: Re: New Linux xfs/reiser file systems

> Matthew Kirkwood <matthew@hairy.beasts.org> writes:
> > From some stracing of 7.1, the most common syscall issued by
> > postgres is an lseek() to the end of the file, presumably to
> > find its length, which seems to happen up to about a dozen
> > times per (pgbench) transaction.
> 
> > Tablespaces would solve this (not that lseek is a particularly
> > expensive operation, of course).
> 
> No, they wouldn't; or at least they'd just create a different problem.
> The reason for the lseek is that the file length may have changed since
> the current backend last checked it.  To avoid lseek we'd need some
> shared data structure that maintains the current length of every active
> table, which would be a nuisance to maintain and probably a source of
> contention delays.

Seems we should cache the file lengths somehow.  Not sure how to do it
because our file system cache is local to each backend.


> (Of course, such a data structure would just be the tip of the iceberg
> of what we'd have to maintain for ourselves if we couldn't depend on the
> kernel to do it for us.  Reimplementing a filesystem doesn't strike me
> as a profitable use of our time.)

Ditto.  The database is complicated enough.

-- 
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