On Thu, 19 Apr 2007, Heikki Linnakangas wrote:

In the sync phase, we sleep between each fsync until enough time/segments have passed, assuming that the time to fsync is proportional to the file length. I'm not sure that's a very good assumption.


I've been making scatter plots of fsync time vs. amount written to the database for a couple of months now, and while there's a trend there it's not a linear one based on data written. Under Linux, to make a useful prediction about how long a fsync will take you first need to consider how much dirty data is already in the OS cache (the "Dirty:" figure in /proc/meminfo) before the write begins, relative to the kernel parameters that control write behavior. Combine that with some knowledge of the caching behavior of the controller/disk combination you're using, and it's just barely possible to make a reasonable estimate. Any less information than all that and you really have very little basis on which to guess how long it's going to take.

Other operating systems are going to give completely different behavior here, which of course makes the problem even worse.

--
* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD

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