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. 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? 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. -- 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
