Re: [PATCH 1/6] fs: add hole punching to fallocate

[Ric Wheeler]

On 01/12/2011 07:44 AM, Dave Chinner wrote:
> On Tue, Jan 11, 2011 at 04:13:42PM -0500, Lawrence Greenfield wrote:
> > On Tue, Nov 9, 2010 at 6:40 PM, Dave Chinner<david@xxxxxxxxxxxxx>  wrote:
> > > The historical reason for such behaviour existing in XFS was that in
> > > 1997 the CPU and IO latency cost of unwritten extent conversion was
> > > significant,
> .....
>
> > > > (Take for example a trusted cluster filesystem backend that checks the
> > > > object checksum before returning any data to the user; and if the
> > > > check fails the cluster file system will try to use some other replica
> > > > stored on some other server.)
> > > IOWs, all they want to do is avoid the unwritten extent conversion
> > > overhead. Time has shown that a bad security/performance tradeoff
> > > decision was made 13 years ago in XFS, so I see little reason to
> > > repeat it for ext4 today....
> > I'd make use of FALLOC_FL_EXPOSE_OLD_DATA. It's not the CPU overhead
> > of extent conversion. It's that extent conversion causes more metadata
> > operations than what you'd have otherwise,
> Yes, that's the "IO latency" part of the cost I mentioned above.
>
> > which means systems that
> > want to use O_DIRECT and make sure the data doesn't go away either
> > have to write O_DIRECT|O_DSYNC or need to call fdatasync().
> Seriously, we tell application writers _all the time_ that they
> *must* use fsync/fdatasync to guarantee their data is on stable
> storage and that they cannot rely on side-effects of filesystem or
> storage specific behaviours (like ext3 ordered mode) to do that job
> for them.
>
> You're suggesting that by introducing FALLOC_FL_EXPOSE_OLD_DATA,
> applications can rely on filesystem/storage specific behaviour to
> guarantee data is on stable storage without the use of
> fdatasync/fsync. Wht you describe is definitely storage specific,
> because volatile write caches still needs the fdatasync to issue a
> cache flush.
>
> Do you see the same conflict here that I do?

The very concept seems quite "non-enterprise".  I also agree that the cost of 
maintaining extra mount options (and code) for something that no sane end user 
would ever do seems to be a loss.

Why wouldn't you want to convert the punched hole to an unwritten extent?

Thanks!

Ric

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