On Fri, Jan 7, 2011 at 3:58 PM, Chris Mason <chris.ma...@oracle.com> wrote: > Excerpts from Olaf van der Spek's message of 2011-01-06 15:01:15 -0500: >> Hi, >> >> Does btrfs support atomic file data replaces? Basically, the atomic >> variant of this: >> // old stage >> open(O_TRUNC) >> write() // 0+ times >> close() >> // new state > > Yes and no. We have a best effort mechanism where we try to guess that > since you've done this truncate and the write that you want the writes > to show up quickly. But its a guess. > > The problem is the write() // 0+ times. The kernel has no idea what > new result you want the file to contain because the application isn't > telling us.
Isn't it safe for the kernel to wait until the first write or close before writing anything to disk? > What btrfs can do (but we haven't yet implemented) is make sure that the > results of a single write file are on disk atomically, even if they are > replacing existing bytes in the file. > > Because we cow and because we don't update metadata pointers until the > IO is complete, we can wait until all the IO for a given write call is > on disk before we update any of the metadata. > > This isn't hard, it's on my TODO list. What about a new flag: O_ATOMIC that'd take the guesswork out of the kernel? Olaf -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
