On Mon, Apr 27, 2015 at 02:41:41PM +0000, sri wrote: > Hugo Mills <hugo <at> carfax.org.uk> writes: > > > > > On Mon, Apr 27, 2015 at 12:05:28PM +0000, sri wrote: > > > Hi, > > > > > > I would like to know if btrfs file system is created on LVM2 logical > > > volumes, does freeze and thaw operations takes place for btrfs to > make > > > file system consistent?? > > > > No, because the filesystem is *always* consistent, because of the > > way that the CoW update mechanism works. > > > > > If not is there a way (ioctl etc..) to run btrfs freeze/thaw before > and > > > after creating LVM2 snapshot of a btrfs file system? > > > > As I said above, none is needed. > > > > HOWEVER, you shouldn't take LVM snapshots of a btrfs filesystem AT > > ALL, if you have the original FS mounted on that system, or if you > > plan on mounting the FS or its snapshots on that system. It's fine if > > the only things mounting it are independent of the base system, and > > don't see the snapshots (e.g. you're exporting LVs as block devices to > > a VM host, and the snapshot is never seen by the VM; or you're > > exporting LVs via NBD, and the remote system never sees the > > snapshot). > > > > If, on the other hand, you're expecting to have /dev/mapper/foo > > mounted, and then take a snapshot of it on the same system, you can > > expect massive filesystem corruption, because the kernel can't > > distinguish between the original block device and the snapshot, and > > may try using the wrong one (or possibly both). > > > > Hugo. > > > > :) Thank you for your input. Past couple of hours, i am unable to figure > it out what is happening with snapshot. > > I am experiencing similar what you have explained. > taken a lvm snapshot using lvcreate of /dev/mapper/foo name > /dev/mapper/foo-snap. > > /dev/mapper/foo is mounted on /b1 > and I have mounted /dev/mapper/foo-snap on /b2 > > after running mount command, i have seen there is a change in the mount > entries. i.e. for both /b1 , /b2 mount point is changed to > /dev/mapper/foo-snap. > > And any write to original or to snap shot are reflecting on both. Correct, this is the behaviour I was referring to above. You're likely to see corrupt filesystems if you do this. > This is strange considering that I wanted a consistent snapshot of > entire btrfs filesystem at volume level. > > Is there a way to achive this? or btrfs just cannot provide? No, there's no way to manage it with the current btrfs code. > I am looking for a way make entire btrfs (which includes subvolumes in > it) consistent so that I can take backup of the file system. You're probably looking for some variant on the recursive snapshots idea. I know at least a couple of people have ideas on how to do this, but I don't think any of the ideas are close to completion. Instead, I would suggest looking at which pieces of your system are *required* to be consistent, and put those on their own subvolume(s). For example, in a multi-user system, I would be very surprised if the individual users' home subvolumes had to be snapshotted as a whole -- nobody will care if person A has their snapshot taken a few seconds (or even half an hour) before person B. Work out what you can get away with non-atomically for your needs, rather than simply requiring a complete atomic snapshot of everything, because if you're using multiple subvolumes, that's simply not going to happen right now. Hugo. -- Hugo Mills | Two things came out of Berkeley in the 1960s: LSD hugo@... carfax.org.uk | and Unix. This is not a coincidence. http://carfax.org.uk/ | PGP: E2AB1DE4 |
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