BTRFS: How big are my snapshots?

Introduction


I have been using BTRFS snapshots for a while now on my laptop to incrementally save the state of my machine before I perform system updates or run some harebrained test. I quickly ran into a problem though, as on a smaller filesystem I was running out of space. I then wanted to be able to look at each snapshot and easily determine how much space I could recover if I deleted each snapshot. Surprisingly this information was not readily available. Of course you could determine the total size of each snapshot by using du, but that only tells you how big the entire snapshot is and not how much of the snapshot is exclusive to this snapshot only..

Enter filesystem quota and qgroups in git commit 89fe5b5f666c247aa3173745fb87c710f3a71a4a . With quota and qgroups (see an overview here ) we can now see how big each of those snapshots are, including exclusive usage.

Steps


The system I am using for this example is Fedora 19 with btrfs-progs-0.20.rc1.20130308git704a08c-1.fc19.x86_64 installed. I have a 2nd disk attached (/dev/sdb) that I will use for the BTRFS filesystem.

First things first lets create a BTRFS filesystem on sdb, mount the filesystem and then create a .snapshots directory.

[root@localhost ~]# mkfs.btrfs /dev/sdb

WARNING! - Btrfs v0.20-rc1 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

fs created label (null) on /dev/sdb
        nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00GB
Btrfs v0.20-rc1
[root@localhost ~]# 
[root@localhost ~]# mount /dev/sdb /btrfs
[root@localhost ~]# mkdir /btrfs/.snapshots


Next lets copy some files into the filesystem. I will copy in a 50M file and then create a snapshot (snap1). Then I will copy in a 4151M file and take another snapshot (snap2). Finally, a 279M file and another snapshot (snap3).

[root@localhost ~]# cp /root/50M_File /btrfs/
[root@localhost ~]# btrfs subvolume snapshot /btrfs /btrfs/.snapshots/snap1
Create a snapshot of '/btrfs' in '/btrfs/.snapshots/snap1'
[root@localhost ~]# 
[root@localhost ~]# cp /root/4151M_File /btrfs/
[root@localhost ~]# btrfs subvolume snapshot /btrfs /btrfs/.snapshots/snap2
Create a snapshot of '/btrfs' in '/btrfs/.snapshots/snap2'
[root@localhost ~]# 
[root@localhost ~]# cp /root/279M_File /btrfs/
[root@localhost ~]# btrfs subvolume snapshot /btrfs /btrfs/.snapshots/snap3
Create a snapshot of '/btrfs' in '/btrfs/.snapshots/snap3'
[root@localhost ~]# 
[root@localhost ~]# df -kh /btrfs/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb         10G  4.4G  3.6G  55% /btrfs


Now how much is each one of those snapshots taking up? We can see this information by enabling quota and then printing out the qgroup information:

[root@localhost ~]# btrfs quota enable /btrfs/
[root@localhost ~]# 
[root@localhost ~]# btrfs qgroup show  /btrfs/
0/5 4698025984 8192
0/257 52432896 4096
0/263 4405821440 12288
0/264 4698025984 8192


The first number on each line represents the subvolume id. The second number represents the amount of space contained within each subvolume (in bytes) and the last number represents the amount of space that is exclusive to that subvolume (in bytes). Now for some reason when I see such large numbers I go brain dead and fail to comprehend how much space is actually being used. I wrote a little perl script to convert the numbers to MB.

[root@localhost ~]# btrfs qgroup show  /btrfs/ | /root/convert
0/5     4480M   0M
0/257   50M     0M
0/263   4201M   0M
0/264   4480M   0M


So that makes sense. The 1st snapshot (denoted by the 2nd line) contains 50M. The 2nd snapshot contains 50M+4151M and the 3rd snapshot contains 50M+4151M+279M. We can also see that at the moment none of them have any exclusive content. This is because all data is shared among them all.

We can fix that by deleting some of the files.

[root@localhost ~]# rm /btrfs/279M_File
rm: remove regular file ‘/btrfs/279M_File’? y
[root@localhost ~]# btrfs qgroup show  /btrfs/ | /root/convert
0/5     4201M   0M
0/257   50M     0M
0/263   4201M   0M
0/264   4480M   278M


Now if we delete all of the files and view the qgroup info, what do we see?

[root@localhost ~]# rm -f /btrfs/4151M_File
[root@localhost ~]# rm -f /btrfs/50M_File
[root@localhost ~]# btrfs qgroup show  /btrfs/ | /root/convert
0/5     0M      0M
0/257   50M     0M
0/263   4201M   0M
0/264   4480M   278M


We can see from the first line that the files have been removed from the root subvolume but the exclusive counts didn’t go up for snap1 and snap2?

This is because the files are shared with snap3. If we remove snap3 then we’ll see the exclusive number go up for snap2:

[root@localhost ~]# btrfs subvolume delete /btrfs/.snapshots/snap3
Delete subvolume '/btrfs/.snapshots/snap3'
[root@localhost ~]#
[root@localhost ~]# btrfs qgroup show  /btrfs/ | /root/convert
0/5     -4480M  -278M
0/257   50M     0M
0/263   4201M   4151M
0/264   4480M   278M


As expected the 2nd snapshot now shows 4151M as exclusive. However, unexpectedly the qgroup for the 3rd snapshot still exists and the root subvolume qgroup now shows negative numbers.

Finally lets delete snap2 and observe that the amount of exclusive space (4151M) is actually released back to the pool of free space:

[root@localhost ~]# df -kh /btrfs/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb         10G  4.2G  3.9G  52% /btrfs
[root@localhost ~]#
[root@localhost ~]# btrfs subvolume delete /btrfs/.snapshots/snap2
Delete subvolume '/btrfs/.snapshots/snap2'
[root@localhost ~]#
[root@localhost ~]# btrfs qgroup show  /btrfs/ | /root/convert
0/5     -8682M  -4430M
0/257   50M     50M
0/263   4201M   4151M
0/264   4480M   278M
[root@localhost ~]#
[root@localhost ~]# df -kh /btrfs/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdb         10G   52M  8.0G   1% /btrfs


So we can see that the space is in fact released and is now counted as free space. Again the negative numbers and the fact that the qgroups show up for the deleted subvolumes is a bit odd.

Cheers!

Dusty Mabe

Bonus: It seems like there is a patch floating around to enhance the output of qgroup show. Check it out here.