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 11:22:08 -0500 Message-ID: <20100212162207.GB4191@localhost.localdomain> References: <12b5f1ef1002111749u4f33b626jb6a901b29f05337f@mail.gmail.com> <93cdabd21002112050x795ab5e2s9bcd426f19032f8c@mail.gmail.com> <20100212151940.GA4191@localhost.localdomain> <93cdabd21002120818g4c47e2b6k3083a368286651e5@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Cc: Josef Bacik , Chris Ball , Nickolai Zeldovich , linux-btrfs@vger.kernel.org To: Mike Fedyk Return-path: In-Reply-To: <93cdabd21002120818g4c47e2b6k3083a368286651e5@mail.gmail.com> List-ID: On Fri, Feb 12, 2010 at 08:18:01AM -0800, Mike Fedyk wrote: > On Fri, Feb 12, 2010 at 7:19 AM, 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= : > >> > =A0 > echo x1 > /mnt/x/d/foo.txt || exit 2 > >> > =A0 > 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 de= fault; > >> > cmason wants the no-sync version of snapshot creation to be avai= lable, > >> > 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 ha= d > >> > understood we were supposed to be running sync first. > >> > > >> You're saying that it only snapshots the on-disk data structures a= nd > >> not the in-memory versions? =A0That can only lead to pain. =A0What= do you > >> do if something else during this race condition? =A0What would a s= ync do > >> to solve this? =A0Have the semantics of sync been changed in btrfs= from > >> "sync everything that hasn't been written yet" to "sync this > >> subvolume"? > >> > > > > Welcome to delalloc. =A0You either get fast writes or you get all o= f your data on > > the disk every 5 seconds. =A0If you don't like delalloc, use ext3. = =A0The 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 ar= ound long > > enough and should be written back to the disk, could happen now, co= uld happen 10 > > years from now. > > 3) sync - this is not as obvious. =A0sync doesn't mean anything tha= n "start > > writing back dirty data to the fs", and returns before it's done. =A0= =46or btrfs > > what that means is we run through _every_ inode that has delalloc p= ages > > associated with them and start writeback on them. =A0This will get = most of your > > data into the current transaction, which is when the snapshot happe= ns. > > > > 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 o= ut quite > > well. =A0Thanks, > > >=20 > Then you broke your ordering guarantee. If the data isn't there, the > meta-data shouldn't be there either. So the snapshots made before th= e > data hits a transaction shouldn't have the file at all. Nope, what is happening is fd =3D creat("file") <- this is metadata that needs to be written write(fd, buf) <- because of delalloc there is no metadata that is= created for this operation, therefore it doesn't need to be written out. close(fd) so the file has metadata created for it, which needs to be written out.= Because of delalloc there are no extents created or anything for the data, ther= efore there is nothing to write. 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