On Fri, Dec 26, 2014 at 03:24:59PM +0000, Ankur Tank wrote: > I wanted to test btrfs on the eMMC of beaglebone black based custom board. > Precondition: eMMC is formatted with ext4 filesystem > Use case: > Format eMMC with mkfs.btrfs -L <label> <dev> > Result: > Mkfs.btrfs denies formatting eMMC because its existing filesystem > > # mkfs.btrfs -L "1storage" /dev/mmcblk0p2 > /dev/mmcblk0p2 appears to contain an existing filesystem (ext4). > Error: Use the -f option to force overwrite. > > If I add "-f" its possible to format the eMMC. It seems to be a feature. mkfs.btrfs will test the device with libblkid to see if there is a filesystem present to prevent administrator mistakes. If libblkid can identify and device, it will prevent format without -f. If you're doing an automated script, you need the -f. libblkid isn't particularly reliable and can false-positive on random data (e.g. the HFS module can detect a filesystem that is not there). If you're doing tests that involve reformatting the filesystem in a loop, you need the -f. > # mkfs.btrfs -f -L "1storage" /dev/mmcblk0p2 > Detected a SSD, turning off metadata duplication. Mkfs with -m dup if you want to force metadata duplication. > Btrfs v3.17 > See http://btrfs.wiki.kernel.org for more information. > > Performing full device TRIM (1.72GiB) ... > Turning ON incompat feature 'extref': increased hardlink limit per file to 65536 > [273917.692896] btrfs: device label 1storage devid 1 transid 3 /dev/mmcblk0p2 > fs created label 1storage on /dev/mmcblk0p2 > nodesize 16384 leafsize 16384 sectorsize 4096 size 1.72GiB > > I had downloaded debian package from following link > https://packages.debian.org/sid/armhf/btrfs-tools/download > > Is it a bug ? or I am missing something ? If your device is small or solid-state, btrfs will change the default options (mixed-metadata, no duplicate metadata). "extref" is a feature that enables large numbers of hardlinks to a single file. You can leave it enabled-it's unlikely your devices will ever see a kernel old enough that it's not compatible.
Attachment:
signature.asc
Description: Digital signature
