Messages in this thread | | | Subject | Re: [PATCH 1/7] block: Add block_flush_device() | From | Chris Mason <> | Date | Tue, 31 Mar 2009 09:16:16 -0400 |
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On Mon, 2009-03-30 at 16:52 -0400, Mark Lord wrote: > Jens Axboe wrote: > > On Mon, Mar 30 2009, Linus Torvalds wrote: > >> > >> On Mon, 30 Mar 2009, Jens Axboe wrote: > >>> Sorry, I just don't see much point to doing it this way instead. So now > >>> the fs will have to check a queue bit after it has issued the flush, how > >>> is that any better than having the 'error' returned directly? > >> No. > >> > >> Now the fs SHOULD NEVER CHECK AT ALL. > >> > >> Either it did the ordering, or the FS cannot do anything about it. > >> > >> That's the point. EOPNOTSUPP is n ot a useful error message. You can't > >> _do_ anything about it. > > > > My point is that some file systems may or may not have different paths > > or optimizations depending on whether barriers are enabled and working > > or not. Apparently that's just reiserfs and Chris says we can remove it, > > so it is probably a moot point. > .. > > XFS appears to have something along those lines. > I believe it tries to disable the drive write caches > if it discovers that it cannot do cache flushes. >
If we get EOPNOTSUPP back from a submit_bh/submit_bio, the IO didn't happen. So, all the filesystems have code to try again without the barrier flag, and then stop doing barriers from then on.
I'm not saying this is a good or bad API, just explaining for this one example how it is being used today ;)
> I'll check next time my MythTV box boots up. > It has a RAID0 under XFS, and the md raid0 code doesn't > appear to pass the cache flushes to libata for raid0, > so XFS complains and tries to turn off the write caches. > > > And I have a script to damn well turn them back ON again > after it does so. Stupid thing tries to override user policy again. >
XFS does print a warning about not doing barriers any more, but the write cache should still be on. Especially with MD in front of it, the storage stack is pretty complex, a mounted filesystem would have a hard time knowing where to start to turn off write caches on each drive in the stack.
You can test this pretty easily:
dd if=/dev/zero of=foo bs=4k count=10000 oflag=direct
If that runs faster than 1MB/s the write cache is still on.
-chris
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