[ZODB-Dev] Use of fsync in FileStorage

[Tim Peters]

[Christian Robottom Reis]
> I made some extra timings with vmstat output for the run on the server I
> reported on before (raid-1 on lvd 10K scsi drives/adaptec 29160):
>
>  No fsyncs: 8.29973 seconds, 1204.86 txn/sec
...
>  One fsync: 11.4103 seconds, 876.397 txn/sec
...
>  Two fsyncs: 12.8292 seconds, 779.474 txn/sec

That's "the other" Linux pattern emerging, which I'm associating with SCSI
drives (because I have to guess *something* <wink>):

- Less than a factor of 2 difference 0-vs-2 fsync (the other Linux pattern
  is ballpark 10x difference here)
- Minor difference 1-vs-2 (in common w/ the other pattern)
- Most of the "damage" done by the first fsync (also in common)

> Doesn't really seem to make *that* much of a difference;

Right, I think even 0-vs-2 is minor on that box.  You're not throwing 45K
txn/minute at your FileStorage in real life (do correct me if I'm wrong
about that, but I'd be majorly surprised if you were), and I bet the 2-fsync
rate is way more than enough for what you do throw at it.  You're already
paying for 1 fsync in FileStorage, and the 1-vs-2 difference is somewhere
between trivial and minor here.

> this may have to do with VFS fsync behaviour when using raid-1, or
> perhaps the write behaviour on these specific hard drives --

Or all of the above, plus more.  We're short on disk details in several of
the other reports.

> I am surprised so many others saw so much fluctuation in times.

Ah, you started using computers this year <wink>?

> I might run this on a similar server that I have remote access to if Tim
> says he wants me to.

If you're still willing, I'm still eager!  Thanks.