On Wed, 30 Nov 2011, Jan Kara wrote: > On Wed 30-11-11 13:05:14, Alasdair G Kergon wrote: > > On Wed, Nov 30, 2011 at 07:14:18AM -0500, Mikulas Patocka wrote: > > > On Wed, 30 Nov 2011, Jan Kara wrote: > > > > > So if you skip sync of frozen filesystems, you introduce a data > > > > > corruption if someone takes a snapshot of ext2. > > > > Yes, because ext2 cannot really be frozen, it is (errorneously) marked > > > > as such but it is not frozen... > > > > This is just getting into semantics. AFAIK (and it was before my involvement) > > LVM used the term 'lockfs' for this operation when it was introduced to ext2. > > It later got renamed in-kernel to 'frozen' to bring it into line with newer > > filesystems. But userspace and the interface still retain the original > > 'lockfs' name. > > > > There is no further I/O sent to the filesystem during the 'lockfs' operation: > > LVM uses dm to block that. > OK, so can we (at least in this discussion) discussion distinguish two > things? > a) Filesystems is frozen/locked - means filesystem is in a consistent state > and disallows new dirty data to be created until fs is thawed/unlocked. Agreed. Note that if you observed any sync-related deadlocks when suspended, it means that the filesystem has some code path that allows creating dirty data on frozen filesystem. This was observed on ext4 on RHEL-6 ... and maybe on upstream too. (I couldn't reproduce it on upstream, but maybe other people who started these sync-related patches could?) > b) Device is frozen/locked - device does not process incoming writes, they > are held in the queue until the device is thawed/unlocked. > > They are two different things and we seem to conflate them in the > discussion. In particular you can freeze a device under any filesystem > while you cannot freeze every filesystem. Freezing the device is enough for > LVM operations (e.g. snapshot) but if filesystem is not frozen, you have > to run fsck / journal replay to make result usable. Do we agree here? > > Honza True. You have to run fsck on non-journaled filesystems when taking a snapshot. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
