Messages in this thread | | | Date | Sun, 29 Mar 2009 14:50:00 +0200 | From | Pavel Machek <> | Subject | Re: replace() system call needed (was Re: EXT4-ish "fixes" in UBIFS) |
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>>> We have a problem that user-space people do not want to >>> use 'fsync()', even when they are pointed to their code >>> which is doing create/write/rename/close without fsync(). >> >> Well... they really don't want to spin the disk up for the >> fsync(). I'm not sure if fsync() is really sensible operation to use >> there. > > I'm personally concerned about hand-held, and in case of UBIFS > fsync is not too expensive - we work on flash and on fsync() we > write back only the stuff belonging to inode in question, and > nothing else.
Well, I'm more concerned about spinning disks, having one even in my zaurus. And I do believe that fsync() will write more data than neccessary even in flash case.
>>> 1. truncate/write/close leads to empty files >> >> this is buggy. > > In FS, or in application?
Application is buggy; no way kernel can help there.
>>> 2. create/write/rename leads to empty files >> >> ..but this should not be. If we want to make that explicit, we should >> provide "replace()" operation; where replace is rename that makes sure >> that source file is completely on media before commiting the rename. > > Well, OK, we can fsync() before rename, we just need clean rules > for this, so that all Linux FSes would follow them. Would be nice > to have final agreement on all this stuff.
My proposal is
rename() stays.
replace(src, bar) is rename that ensures that bar will contain valid data after powerfail.
>> It is somehow similar to fsync()/rename(), but does not force disk >> spin up immediately -- it only inserts "barrier" between data blocks >> and rename. (And yes, it should be implemented as fsync()+rename() for >> filesystems like xfs. It can be implemented as plain rename for ext3 >> and ext4 after the fixes...) > > Right. But I guess only few file-systems would really implement > this, because this is complex.
Complex yes, but at least ext3+ext4+btrfs should, and they really have 90% of "market share" :-). ext3 and ext4 implementations are already done :-). Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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