From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josef Bacik Subject: Re: zero-length files in snapshots Date: Fri, 12 Feb 2010 13:45:11 -0500 Message-ID: <20100212184511.GD4191@localhost.localdomain> References: <12b5f1ef1002111749u4f33b626jb6a901b29f05337f@mail.gmail.com> <93cdabd21002112050x795ab5e2s9bcd426f19032f8c@mail.gmail.com> <20100212151940.GA4191@localhost.localdomain> <4B759C54.8050907@p-static.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-btrfs@vger.kernel.org To: Ravi Pinjala Return-path: In-Reply-To: <4B759C54.8050907@p-static.net> List-ID: On Fri, Feb 12, 2010 at 12:22:12PM -0600, Ravi Pinjala wrote: > On 02/12/10 09:19, Josef Bacik wrote: >> On Thu, Feb 11, 2010 at 08:50:48PM -0800, Mike Fedyk wrote: >>> On Thu, Feb 11, 2010 at 7:11 PM, Chris Ball wrote: >>>> > echo x1> /mnt/x/d/foo.txt || exit 2 >>>> > btrfsctl -s /mnt/x/snap /mnt/x/d >>>> >>>> You're just missing a sync/fsync() between these two lines. >>>> >>>> We argued on IRC a while ago about whether this is a sensible default; >>>> cmason wants the no-sync version of snapshot creation to be available, >>>> but was amenable to the idea of changing the default to be sync before >>>> snapshot, since it was pointed out that no-one other than him had >>>> understood we were supposed to be running sync first. >>>> >>> You're saying that it only snapshots the on-disk data structures and >>> not the in-memory versions? That can only lead to pain. What do you >>> do if something else during this race condition? What would a sync do >>> to solve this? Have the semantics of sync been changed in btrfs from >>> "sync everything that hasn't been written yet" to "sync this >>> subvolume"? >>> >> >> Welcome to delalloc. You either get fast writes or you get all of your data on >> the disk every 5 seconds. If you don't like delalloc, use ext3. The data >> you've written to memory doesn't go down to disk unless explicitly told to, such >> as >> >> 1) fsync - this is obvious >> 2) vm - the vm has decided that this dirty page has been sitting around long >> enough and should be written back to the disk, could happen now, could happen 10 >> years from now. >> 3) sync - this is not as obvious. sync doesn't mean anything than "start >> writing back dirty data to the fs", and returns before it's done. For btrfs >> what that means is we run through _every_ inode that has delalloc pages >> associated with them and start writeback on them. This will get most of your >> data into the current transaction, which is when the snapshot happens. >> >> If you don't want empty files, do something like this >> >> btrfsctl -c /dir/to/volume >> btrfsctl -s /dir/to/volume/snapshotname /dir/to/volume >> >> this is what we do with yum and its rollback plugin, and it works out quite >> well. Thanks, >> >> Josef >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > > Is there a race in there? It seems like if a process starts modifying a > file between the sync and the snapshot, data could still be lost. Is > there something else going on here that I'm missing that would prevent > this race? > Data won't be lost, it just won't be there in the snapshot, and will be there in the source. Thanks, Josef