Messages in this thread | | | Date | Sun, 14 May 2006 04:22:39 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 008 of 8] md/bitmap: Change md/bitmap file handling to use bmap to file blocks. |
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Neil Brown <neilb@suse.de> wrote: > > On Saturday May 13, akpm@osdl.org wrote: > > Paul Clements <paul.clements@steeleye.com> wrote: > > > > > > Andrew Morton wrote: > > > > > > > The loss of pagecache coherency seems sad. I assume there's never a > > > > requirement for userspace to read this file. > > > > > > Actually, there is. mdadm reads the bitmap file, so that would be > > > broken. Also, it's just useful for a user to be able to read the bitmap > > > (od -x, or similar) to figure out approximately how much more he's got > > > to resync to get an array in-sync. Other than reading the bitmap file, I > > > don't know of any way to determine that. > > > > Read it with O_DIRECT :( > > Which is exactly what the next release of mdadm does. > As the patch comment said: > > : With this approach the pagecache may contain data which is inconsistent with > : what is on disk. To alleviate the problems this can cause, md invalidates > : the pagecache when releasing the file. If the file is to be examined > : while the array is active (a non-critical but occasionally useful function), > : O_DIRECT io must be used. And new version of mdadm will have support for this.
Which doesn't help `od -x' and is going to cause older mdadm userspace to mysteriously and subtly fail. Or does the user<->kernel interface have versioning which will prevent this?
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