Problem FAQ

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How do I report bugs and issues?

Please report bugs and issues to the mailing list (; you are not required to subscribe). For quick questions you may want to join the IRC #btrfs channel on freenode (and stay around for some time in case you do not get the answer right away).

(Optionally you can use the bugzilla on Never use the bugzilla on "original" Btrfs project page at Oracle.)

I can't mount my filesystem, and I get a kernel oops!

First, update your kernel to the latest one available and try mounting again. If you have your kernel on a btrfs filesystem, then you will probably have to find a recovery disk with a recent kernel on it.

Second, try mounting with options -o recovery or -o ro or -o recovery,ro (using the new kernel). One of these may work successfully.

Finally, if and only if the kernel oops in your logs has something like this in the middle of it, then it's probably fixable easily:

? replay_one_dir_item+0xb5/0xb5 [btrfs]
? walk_log_tree+0x9c/0x19d [btrfs]
? btrfs_read_fs_root_no_radix+0x169/0x1a1 [btrfs]
? btrfs_recover_log_trees+0x195/0x29c [btrfs]
? replay_one_dir_item+0xb5/0xb5 [btrfs]
? btree_read_extent_buffer_pages+0x76/0xbc [btrfs]
? open_ctree+0xff6/0x132c [btrfs]

If you have errors like this, then your log tree is probably corrupt, and removing it will allow you to mount the filesystem again. The common case where this happened has been fixed a long time ago, so it is unlikely that you will see this particular problem. You will lose the last few seconds of activity on the filesystem from before your original corruption (up to 30 seconds), but you will at least have a mountable FS. If you are not sure whether you have a situation that these instructions apply to, please ask on the mailing list or on IRC.

You will need to build and run the btrfs-zero-log tool: get a recent copy of the user-space tools, and build them:

$ make
$ make btrfs-zero-log
$ sudo btrfs-zero-log /dev/sda1

(replacing /dev/sda1 with whatever device(s) your FS resides on)

Running btrfs-zero-log on a filesystem with any other kind of mount problem will most likely not fix it, and may make recovering it harder. If in doubt, check with the developers on IRC or the mailing list first.

Filesystem can't be mounted by label

See the next section.

Only one disk of a multi-volume filesystem will mount

If you have labelled your filesystem and put it in /etc/fstab, but you get:

# mount LABEL=foo
mount: wrong fs type, bad option, bad superblock on /dev/sdd2,
      missing codepage or helper program, or other error
      In some cases useful info is found in syslog - try
      dmesg | tail  or so

or if one volume of a multi-volume filesystem fails when mounting, but the other succeeds:

# mount /dev/sda1 /mnt/fs
mount: wrong fs type, bad option, bad superblock on /dev/sdd2,
      missing codepage or helper program, or other error
      In some cases useful info is found in syslog - try
      dmesg | tail  or so
# mount /dev/sdb1 /mnt/fs

Then you need to ensure that you run a btrfs device scan first:

# btrfs device scan

This should be in many distributions' startup scripts (and initrd images, if your root filesystem is btrfs), but you may have to add it yourself.

My filesystem won't mount and none of the above helped. Is there any hope for my data?

Maybe. Any number of things might be wrong. The restore tool is a non-destructive way to dump data to a backup drive and may be able to recover some or all of your data, even if we can't save the existing filesystem.

Defragmenting a directory doesn't work

Running this:

# btrfs filesystem defragment ~/stuff

does not defragment the contents of the directory.

This is by design. btrfs fi defrag operates on the single filesystem object passed to it. This means that the command defragments just the metadata held by the directory object, and not the contents of the directory. If you want to defragment the contents of the directory, something like this would be more useful:

# find -xdev -type f -exec btrfs fi defrag '{}' \;

Caveat: Defragmenting a file which has a COW copy (either a snapshot copy or one made with cp --reflink or bcp) will produce two unrelated files. If you defragment a subvolume that has a snapshot, you will roughly double the disk usage, as the snapshot files are no longer COW images of the originals.

Compression doesn't work / poor compression ratios

First of all make sure you have passed "compress" mount option in fstab or mount command. If yes, and ratios are unsatisfactory, then you might try "compress-force" option. This way you make the btrfs to compress everything. The reason why "compress" ratios are so low is because btrfs very easily backs out of compress decision. (Probably not to waste too much CPU time on bad compressing data).

Copy-on-write doesn't work

You've just copied a large file, but still it consumed free space. Try:

# cp --reflink=always file1 file2

I get the message "failed to open /dev/btrfs-control skipping device registration" from "btrfs dev scan"

You are missing the /dev/btrfs-control device node. This is usually set up by udev. However, if you are not using udev, you will need to create it yourself:

# mknod /dev/btrfs-control c 10 234

You might also want to report to your distribution that their configuration without udev is missing this device.

How to clean up old superblock ?

The preferred way is to use the wipefs utility that is part of the util-linux package. Running the command with the device will not destroy the data, just list the detected filesystems:

# wipefs /dev/sda
offset               type
0x10040              btrfs   [filesystem]
                     UUID:  7760469b-1704-487e-9b96-7d7a57d218a5

To actually remove the filesystem use:

# wipefs -o 0x10040 /dev/sda
8 bytes [5f 42 48 52 66 53 5f 4d] erased at offset 0x10040 (btrfs)

ie. copy the offset number to the commandline parameter.

Note: The process is reversible, if the 8 bytes are written back, the device is recognized again.
Note: wipefs clears only the first superblock. If the first superblock is further invalidaded the other ones could "resurrect" the filesystem.

Related problem:

Long time ago I created btrfs on /dev/sda. After some changes btrfs moved to /dev/sda1.

Use wipefs as well, it deletes only a small portion of sda that will not interfere with the next partition data.

What if I don't have wipefs at hand?

There are three superblocks: the first one is located at 64K, the second one at 64M, the third one at 256GB. The following lines reset the magic string on all the three superblocks

# dd if=/dev/zero bs=1 count=8 of=/dev/sda seek=$((64*1024+64))
# dd if=/dev/zero bs=1 count=8 of=/dev/sda seek=$((64*1024*1024+64))
# dd if=/dev/zero bs=1 count=8 of=/dev/sda seek=$((256*1024*1024*1024+64))

If you want to restore the superblocks magic string,

# echo "_BHRfS_M" | dd bs=1 count=8 of=/dev/sda seek=$((64*1024+64))
# echo "_BHRfS_M" | dd bs=1 count=8 of=/dev/sda seek=$((64*1024*1024+64))
# echo "_BHRfS_M" | dd bs=1 count=8 of=/dev/sda seek=$((256*1024*1024*1024+64))

I get "No space left on device" errors, but df says I've got lots of space

First, check how much space has been allocated on your filesystem:

$ sudo btrfs fi show
Label: 'media'  uuid: 3993e50e-a926-48a4-867f-36b53d924c35
	Total devices 1 FS bytes used 61.61GB
	devid    1 size 133.04GB used 133.04GB path /dev/sdf

Note that in this case, all of the devices (the only device) in the filesystem are fully utilised. This is your first clue.

Next, check how much of your metadata allocation has been used up:

$ sudo btrfs fi df /mount/point
Data: total=127.01GB, used=56.97GB
System, DUP: total=8.00MB, used=20.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=3.00GB, used=2.32GB
Metadata: total=8.00MB, used=0.00

Note that the Metadata used value is fairly close (75% or more) to the Metadata total value, but there's lots of Data space left. What has happened is that the filesystem has allocated all of the available space to either data or metadata, and then one of those has filled up (usually, it's the metadata space that does this). For now, a workaround is to run a partial balance:

$ sudo btrfs balance start -dusage=5 /mount/point

Note that there should be no space between the -d and the usage. This command will attempt to relocate data in empty or near-empty data chunks, allowing the space to be reclaimed and reassigned to metadata. More information is available elsewhere in this wiki, if you want to know what a balance does, or what options are available for the balance command.

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