Messages in this thread | | | Date | Fri, 28 Jan 2011 13:13:16 -0500 | From | Ric Wheeler <> | Subject | Re: [PATCH 1/6] fs: add hole punching to fallocate |
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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@fromorbit.com> 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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