On 04/12/2011 10:39 PM, Eric Sandeen wrote: > On 4/12/11 1:04 PM, Johann Lombardi wrote: >> Prevent an ext4 filesystem from being mounted multiple times. A >> sequence number is stored on disk and is periodically updated >> (every 5 seconds by default) by a mounted filesystem. At mount >> time, we now wait for s_mmp_update_interval seconds to make sure >> that the MMP sequence does not change. In case of failure, the >> nodename, bdevname and the time at which the MMP block was last >> updated is displayed. >> >> Signed-off-by: Andreas Dilger <adilger@xxxxxxxxxxxxx> >> Signed-off-by: Johann Lombardi <johann@xxxxxxxxxxxxx> --- >> fs/ext4/ext4.h | 56 ++++++++- fs/ext4/super.c | 363 >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++- 2 files >> changed, 416 insertions(+), 3 deletions(-) >> > > There was a lot of skepticism about this last time, and I imagine > there still is... > > 400 new lines of kernel code for this, and if the other machine is > hung up for 5 seconds and doesn't update, it can still be > multiply-mounted anyway, right? > > BUG: soft lockup - CPU#0 stuck for 10s! anyone? :( Please see my other comment about the two different intervals. Yes, there is a minimal chance of a race. But firstly, 5s are too small, already for performance reasons (setting the update-interval to 5s will increase the min check-interval to 25s). Secondly, the mount-wait time is + wait_time = min(mmp_check_interval * 2 + 1, + mmp_check_interval + 60); So even with Johanns patch it is at least 12s. Thirdly, the check-interval is automatically increased, if updating the mmp block takes too long. This value will also be saved in the mmp-block. Of course, it has a disadvantage - the mount time increases. > > I don't see the value in it for upstream ext4, but then hey, ext4 > rarely meets a feature it doesn't like ;) Is ext4 is only used on desktop systems? IMHO, every HA solution that does not use scsi reservations or another way to check if a device is already in use, needs a solution like this. I have seen so many problems with heartbeat/pacemaker to not properly detect an already mounted devices (*) and this MMP patch already protected so many HA Lustre installations from data corruption due to double mounts.... So why shouldn't other HA solutions benefit from such a nice feature? Usually, the heartbeat/pacemaker issues to detect if a device is mounted or not are due to unreliable information if a device is mounted or not. /etc/mtab is entirely unreliable and /proc/mounts does not always show if a device is mounted or not. However, even if that would work somehow perfectly, without the MMP patch there is still zero protection from user-errors. It can easily happen an admin forgets about a mounted device and runs e2fsck or manually mounts the device on another machine again. So please, let this patch go in. Thanks, Bernd -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
