Re: [Bug 421482] Firefox 3 uses fsync excessively

[Jamie Lokier <jamie@shareable.org>]

Jörn Engel wrote:
> > We're currently forcing a new commit if I_DIRTY_SYNC or
> > I_DIRTY_DATASYNC (but not necessarily I_DIRTY_PAGES) is set.  If
> > I_DIRTY_SYNC really means "smudged" (I believe you but I'll want to go
> > through the code and prove it to myself :-),
> 
> Proving it to yourself is good advice indeed.  I'm sure it used to mean
> "smudged" in 2.4.0 time.  Whether any changes since have damaged that
> property I haven't checked.

I noticed fdatasync() doing a full fsync(), and had a look at those
flags a few kernels ago, to implement fdatasync().  I wasn't convinced
the flags were being used in that way, but now I don't remember why.

So, yes, do check what they mean _now_.  And then, please, make us all
happy and implement fdatasync() :-)

Here's a thought for someone implementing fdatasync().  If a database
uses O_DIRECT writes (typically with aio), then wants data which it's
written to be committed to the hard disk platter, and the filesystem
is mounted "barrier=1" - should it call fdatasync()?  Should that emit
the barrier?  If another application uses normal (not O_DIRECT)
writes, and then _is delayed_ so long that kernel writeback occurs and
all cache is clean, and then calls fdatasync(), should that call emit
a barrier in that case?  (Answers imho: yes and yes).

> > (And then, we should rename things to more sane names.  :-)

Please, yes!  The names made sense instinctively, until I looked at
the code then they didn't :-)

-- Jamie
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