Hi On Fri, Jan 9, 2015 at 9:22 PM, Lukáš Czerner <lczerner@xxxxxxxxxx> wrote: > On Fri, 9 Jan 2015, Stefan Hajnoczi wrote: > >> Date: Fri, 9 Jan 2015 12:01:59 +0000 >> From: Stefan Hajnoczi <stefanha@xxxxxxxxxx> >> To: Al Viro <viro@xxxxxxxxxxxxxxxxxx> >> Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>, >> "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, >> 'linux-ext4' <linux-ext4@xxxxxxxxxxxxxxx> >> Subject: Re: Can one file system be R/W mounted multiple times in Linux? >> >> On Fri, Jan 09, 2015 at 03:51:23AM +0000, Al Viro wrote: >> > On Fri, Jan 09, 2015 at 03:44:16AM +0000, Dexuan Cui wrote: >> > > The 'mount' utility allows me to mount 1 file systems multiple times at different >> > > mount points, like "mount /dev/sdb1 /a; mount /dev/sdb1 /b". >> > > >> > > I tried to write from /a and /b at the same time and it seems everything is OK >> > > and no data corruption happens. >> > > I tried only ext4 only. >> > > >> > > Can somebody please tell me if this usage is safe? >> > >> > Yes. >> >> Can you explain under what conditions mounting the same file system >> twice will work? >> >> I guess the kernel is looking up the block device and then sharing the >> superblock if the file system is already mounted on this block device? >> >> If I use two loop devices for the same underlying storage it does not >> work: >> >> # mount -o loop /var/tmp/ext4.img /tmp/a >> # mount -o loop /var/tmp/ext4.img /tmp/b > > Because now you have two block devices backed by the same file. > Which as Christoph pointed out is equivalent of accessing it from > two different systems. > > What you probably want to do is this: > > # losetup --show -f /var/tmp/ext4.img > /dev/loop0 > > mount /dev/loop0 /tmp/a > mount /dev/loop0 /tmp/b > > Now the question is, whether 'mount' can be a bit smarter than that > and just mount the already existing block device instead of creating > new one ? Karel ? It's not the mount. If you see mount_bdev it checks for this and uses the existing super block from the first mount for all subsequent mounts of that block device. When block devices differ, as in with 2 loop devices they have different bdev and that's why it doesn't work with 2 loop devices. > > -Lukas > > >> # touch /tmp/a/a >> # ls /tmp/b >> lost+found >> # umount /tmp/a >> # umount /tmp/b >> # mount -o loop /var/tmp/ext4.img /tmp/a >> [1078357.297245] EXT4-fs error (device loop0): ext4_lookup:1441: inode #2: comm ls: deleted inode referenced: 12 >> >> Stefan >> > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- ---P.K.S -- 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
