Re: vim file write mode on journaling fs.

[Eric Sandeen <sandeen@sgi.com>]

Colin Walters wrote:
> 
> Steve Lord <lord@sgi.com> writes:
> 
> >   2. An individual thread doing a write in XFS has no way of knowing
> >   or predicting what else may just of happened, or be about to
> >   happen on the system. You cannot say 'I will write my data now
> >   because the system is idle', there may be a couple of Gbytes of
> >   I/O about to come in from another source.
> 
> Why couldn't the kernel keep some statistics about disk usage, and use
> those to make an educated guess as to whether or not we should write
> out the data immediately?
> 
> I appreciate that XFS can deal with gigabytes of I/O, but such an
> event is very unlikely on my laptop :)

Again, this really isn't XFS-specific. The bdflush daemon pushes out
metadata every 5 seconds, data every 30 (by default).  You'll see this
same behavior with ext2.

I think a big reason that people are seeing it "more" with XFS is that
after recovery, a file exists with "nulls" in it, which looks like
corruption.  Based on the quick experiment in my previous email, I think
the difference is that with ext2, the user would never even see the
file, so they may not notice.  "Blissfully unaware" as they say.  :)

Regarding prediction for when to write, that's a whole different
ballpark.  It would be pretty tough to "predict" that there will be no
other I/O's happening in the next 2 seconds, so now is a good time to
write that 5 byte file...

-Eric

-- 
Eric Sandeen      XFS for Linux     http://oss.sgi.com/projects/xfs
sandeen@sgi.com   SGI, Inc.