NAS – Next-gen filesystems – BTRFS RAID 5

Posted on by enira

Introduction

This part continues on the previous post. Except I’ve loaded up some extra virtual disks for testing RAID5. With BTRFS RAID5 is the most experimental part. With being just implemented in kernel 3.19.

The tests

Test One: Setting up the RAID5

So let’s look at our setup. In this case I have three 6GB disks.

lsblk
NAME                        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                           8:0    0     6G  0 disk
├─sda1                        8:1    0   243M  0 part /boot
├─sda2                        8:2    0     1K  0 part
└─sda5                        8:5    0   5.8G  0 part
  ├─btrfs--vg-root (dm-0)   252:0    0   4.8G  0 lvm  /
  └─btrfs--vg-swap_1 (dm-1) 252:1    0     1G  0 lvm  [SWAP]
sdb                           8:16   0     6G  0 disk
sdc                           8:32   0     6G  0 disk
sdd                           8:48   0     6G  0 disk

So let’s create a BTRFS RAID5 on top of those three.

sudo mkfs.btrfs -d raid5 -m raid5 -L disk-raid5 /dev/sdb /dev/sdc /dev/sdd
sudo btrfs fi show
Label: 'disk-raid5'  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 3 FS bytes used 112.00KiB
        devid    1 size 6.00GiB used 1.23GiB path /dev/sdb
        devid    2 size 6.00GiB used 1.21GiB path /dev/sdc
        devid    3 size 6.00GiB used 1.21GiB path /dev/sdd

As you can see all devices are connected and assigned to the raid5.

Now let’s add it to our fstab file.

nano /etc/fstab

Use the UUID ‘5e8d29ae-aea8-4460-a049-fae62e9994fd’ from the ‘fi show’ command.

UUID=5e8d29ae-aea8-4460-a049-fae62e9994fd /media/btrfs-raid5          btrfs defaults 0       0

Now create our mountpiunt.

sudo mkdir -p /media/btrfs-raid5

And test is all works with a reboot.

sudo reboot

After rebooting we should see our new filesystem. Note that it will show 18GB instead of 12GB. The program ‘df’ has a terrible way to report space used for BTRFS filesystems.

df -h
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/btrfs--vg-root  4.6G  1.9G  2.5G  43% /
none                        4.0K     0  4.0K   0% /sys/fs/cgroup
udev                        477M  4.0K  477M   1% /dev
tmpfs                        98M  984K   97M   1% /run
none                        5.0M     0  5.0M   0% /run/lock
none                        488M     0  488M   0% /run/shm
none                        100M     0  100M   0% /run/user
/dev/sdb                     18G   17M   16G   1% /media/btrfs-raid5
/dev/sda1                   236M  100M  124M  45% /boot

Enabling Samba

Add our ‘/media/btrfs-raid5’ as a samba share so we can add some files.

sudo nano /etc/samba/smb.conf
[btrfs-raid5]
   comment = Test BTRFS RAID 5
   browseable = yes
   path = /media/btrfs-raid5
   valid users = btrfs
   writable = yes

Assign correct rights

sudo chown -R btrfs:btrfs /media/btrfs-raid5/

Restart the service to apply our changes

sudo service smbd restart

Now we can copy some test data to the disks

df -h
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/btrfs--vg-root  4.6G  1.9G  2.5G  44% /
none                        4.0K     0  4.0K   0% /sys/fs/cgroup
udev                        477M  4.0K  477M   1% /dev
tmpfs                        98M  1.3M   97M   2% /run
none                        5.0M     0  5.0M   0% /run/lock
none                        488M     0  488M   0% /run/shm
none                        100M     0  100M   0% /run/user
/dev/sdb                     18G  5.3G  7.6G  42% /media/btrfs-raid5
/dev/sda1                   236M  100M  124M  45% /boot
sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 3 FS bytes used 5.28GiB
        devid    1 size 6.00GiB used 3.95GiB path /dev/sdb
        devid    2 size 6.00GiB used 3.93GiB path /dev/sdc
        devid    3 size 6.00GiB used 3.93GiB path /dev/sdd

Test Two: Expanding the RAID5

For this test I’ve shut down the machine and added an extra disk ‘sde’.

lsblk
NAME                        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                           8:0    0     6G  0 disk
├─sda1                        8:1    0   243M  0 part /boot
├─sda2                        8:2    0     1K  0 part
└─sda5                        8:5    0   5.8G  0 part
  ├─btrfs--vg-root (dm-0)   252:0    0   4.8G  0 lvm  /
  └─btrfs--vg-swap_1 (dm-1) 252:1    0     1G  0 lvm  [SWAP]
sdb                           8:16   0     6G  0 disk
sdc                           8:32   0     6G  0 disk
sdd                           8:48   0     6G  0 disk
sde                           8:64   0     6G  0 disk

Next add it to the mount point ‘/media/btrfs-raid5’. In our case we can add the device also to sdb. (As reported by df to be the disk for this mountpoint.)

sudo btrfs device add /dev/sde /media/btrfs-raid5

Once added you can query the filesystem to see if it’s there.

sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    1 size 6.00GiB used 3.95GiB path /dev/sdb
        devid    2 size 6.00GiB used 3.93GiB path /dev/sdc
        devid    3 size 6.00GiB used 3.93GiB path /dev/sdd
        devid    4 size 6.00GiB used 0.00 path /dev/sde

What is noticable is that there isn’t any data on this disk once we add it. Do note that in BTRFS you are responsible for balancing out your RAID5 once you start adding disks. Before we balance let’s verify the md5sums so we can see if the balance did any harm.

md5sum /media/btrfs-raid5/*
03486548bc7b0f1a3881dc00c0f8c5f8  /media/btrfs-raid5/S01E01 FLEMISH HDTV x264.mp4
a9390aed84a6be8c145046772296db26  /media/btrfs-raid5/S01E02 FLEMISH HDTV x264.mp4
2e37ed514579ac282986efd78ac3bb76  /media/btrfs-raid5/S01E03 FLEMISH HDTV x264.mp4
1596a5e56f14c843b5c27e2d3ff27ebd  /media/btrfs-raid5/S01E04 FLEMISH HDTV x264.mp4
f7d494d6858391ac5c312d141d9ee0e5  /media/btrfs-raid5/S01E05 FLEMISH HDTV x264.mp4
fe6f097ff136428bfc3e2a1b8e420e4e  /media/btrfs-raid5/S01E06 FLEMISH HDTV x264.mp4
43c5314079f08570f6bb24b5d6fde101  /media/btrfs-raid5/S01E07 FLEMISH HDTV x264.mp4
3b5ea952b632bbc58f608d64667cd2a1  /media/btrfs-raid5/S01E08 FLEMISH HDTV x264.mp4
db6b8bf608de2008455b462e76b0c1dd  /media/btrfs-raid5/S01E09 FLEMISH HDTV x264.mp4
0d5775373e1168feeef99889a1d8fe0a  /media/btrfs-raid5/S01E10 FLEMISH HDTV x264.mp4
8dd4b25c249778f197fdb33604fdb998  /media/btrfs-raid5/S01E11 FLEMISH HDTV x264.mp4
edac6a857b137136a4d27bf6926e1287  /media/btrfs-raid5/S01E12 FLEMISH HDTV x264.mp4

Next start the balance.

sudo btrfs balance start /media/btrfs-raid5

Once done we should see that the data is evenly spread across the disks.

sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    1 size 6.00GiB used 2.56GiB path /dev/sdb
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    3 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    4 size 6.00GiB used 2.56GiB path /dev/sde

Now let’s generate our md5sums again to see if the data is changed.

md5sum /media/btrfs-raid5/*
03486548bc7b0f1a3881dc00c0f8c5f8  /media/btrfs-raid5/S01E01 FLEMISH HDTV x264.mp4
a9390aed84a6be8c145046772296db26  /media/btrfs-raid5/S01E02 FLEMISH HDTV x264.mp4
2e37ed514579ac282986efd78ac3bb76  /media/btrfs-raid5/S01E03 FLEMISH HDTV x264.mp4
1596a5e56f14c843b5c27e2d3ff27ebd  /media/btrfs-raid5/S01E04 FLEMISH HDTV x264.mp4
f7d494d6858391ac5c312d141d9ee0e5  /media/btrfs-raid5/S01E05 FLEMISH HDTV x264.mp4
fe6f097ff136428bfc3e2a1b8e420e4e  /media/btrfs-raid5/S01E06 FLEMISH HDTV x264.mp4
43c5314079f08570f6bb24b5d6fde101  /media/btrfs-raid5/S01E07 FLEMISH HDTV x264.mp4
3b5ea952b632bbc58f608d64667cd2a1  /media/btrfs-raid5/S01E08 FLEMISH HDTV x264.mp4
db6b8bf608de2008455b462e76b0c1dd  /media/btrfs-raid5/S01E09 FLEMISH HDTV x264.mp4
0d5775373e1168feeef99889a1d8fe0a  /media/btrfs-raid5/S01E10 FLEMISH HDTV x264.mp4
8dd4b25c249778f197fdb33604fdb998  /media/btrfs-raid5/S01E11 FLEMISH HDTV x264.mp4
edac6a857b137136a4d27bf6926e1287  /media/btrfs-raid5/S01E12 FLEMISH HDTV x264.mp4

Still all good.

Test Three: Replacing a disk

So let’s replace our our first disk sdb. I chose this disk as it represents the mount point in the df command. Ideal test case to break.

NAME                        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                           8:0    0     6G  0 disk
├─sda1                        8:1    0   243M  0 part /boot
├─sda2                        8:2    0     1K  0 part
└─sda5                        8:5    0   5.8G  0 part
  ├─btrfs--vg-root (dm-0)   252:0    0   4.8G  0 lvm  /
  └─btrfs--vg-swap_1 (dm-1) 252:1    0     1G  0 lvm  [SWAP]
sdb                           8:16   0     6G  0 disk
sdc                           8:32   0     6G  0 disk
sdd                           8:48   0     6G  0 disk
sde                           8:64   0     6G  0 disk
sdf                           8:80   0     6G  0 disk

For RAID5 you can add and remove. Although I would recommend replace. But as it is the aim to see if everything breaks, I am going ahead with the add / delete option.

sudo btrfs device add /dev/sdf /media/btrfs-raid5
sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 5 FS bytes used 5.28GiB
        devid    1 size 6.00GiB used 2.56GiB path /dev/sdb
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    3 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    4 size 6.00GiB used 2.56GiB path /dev/sde
        devid    5 size 6.00GiB used 0.00 path /dev/sdf

Now delete the old /dev/sdb.

sudo btrfs device delete /dev/sdb /media/btrfs-raid5

This event causes a rebalance. As the data won’t be redundant. Note: this could take a long time.

sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    3 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    4 size 6.00GiB used 2.56GiB path /dev/sde
        devid    5 size 6.00GiB used 2.56GiB path /dev/sdf

Ok, now we are done generate the md5sums again.

md5sum /media/btrfs-raid5/*
03486548bc7b0f1a3881dc00c0f8c5f8  /media/btrfs-raid5/S01E01 FLEMISH HDTV x264.mp4
a9390aed84a6be8c145046772296db26  /media/btrfs-raid5/S01E02 FLEMISH HDTV x264.mp4
2e37ed514579ac282986efd78ac3bb76  /media/btrfs-raid5/S01E03 FLEMISH HDTV x264.mp4
1596a5e56f14c843b5c27e2d3ff27ebd  /media/btrfs-raid5/S01E04 FLEMISH HDTV x264.mp4
f7d494d6858391ac5c312d141d9ee0e5  /media/btrfs-raid5/S01E05 FLEMISH HDTV x264.mp4
fe6f097ff136428bfc3e2a1b8e420e4e  /media/btrfs-raid5/S01E06 FLEMISH HDTV x264.mp4
43c5314079f08570f6bb24b5d6fde101  /media/btrfs-raid5/S01E07 FLEMISH HDTV x264.mp4
3b5ea952b632bbc58f608d64667cd2a1  /media/btrfs-raid5/S01E08 FLEMISH HDTV x264.mp4
db6b8bf608de2008455b462e76b0c1dd  /media/btrfs-raid5/S01E09 FLEMISH HDTV x264.mp4
0d5775373e1168feeef99889a1d8fe0a  /media/btrfs-raid5/S01E10 FLEMISH HDTV x264.mp4
8dd4b25c249778f197fdb33604fdb998  /media/btrfs-raid5/S01E11 FLEMISH HDTV x264.mp4
edac6a857b137136a4d27bf6926e1287  /media/btrfs-raid5/S01E12 FLEMISH HDTV x264.mp4

To see if my weird changes keep working I want to see if the drive comes back up once deleted.

sudo reboot

Yup, here it is again.

sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    3 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    4 size 6.00GiB used 2.56GiB path /dev/sde
        devid    5 size 6.00GiB used 2.56GiB path /dev/sdf

Test Four: Crashing a disk

In this case I physically (or virtually) disconnected a disk.
So when booting you will see the ‘An error occured while mounting /media/btrfs-raid5’. Press s to skip.

Let’s verify the filesystem.

lsblk
NAME                        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                           8:0    0     6G  0 disk
├─sda1                        8:1    0   243M  0 part /boot
├─sda2                        8:2    0     1K  0 part
└─sda5                        8:5    0   5.8G  0 part
  ├─btrfs--vg-root (dm-0)   252:0    0   4.8G  0 lvm  /
  └─btrfs--vg-swap_1 (dm-1) 252:1    0     1G  0 lvm  [SWAP]
sdb                           8:16   0     6G  0 disk
sdc                           8:32   0     6G  0 disk
sdd                           8:48   0     6G  0 disk
sde                           8:64   0     6G  0 disk

So in our case we disconnected disk ‘sdf’ but this is a false report (see later). Let’s verify if it is still mounted (shouldn’t be there, as we said skip).

df -h
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/btrfs--vg-root  4.6G  1.9G  2.5G  44% /
none                        4.0K     0  4.0K   0% /sys/fs/cgroup
udev                        477M  4.0K  477M   1% /dev
tmpfs                        98M  1.2M   97M   2% /run
none                        5.0M     0  5.0M   0% /run/lock
none                        488M     0  488M   0% /run/shm
none                        100M     0  100M   0% /run/user
/dev/sda1                   236M  100M  124M  45% /boot

Now let’s inspect our BTRFS filesystem.

sudo btrfs fi show
Label: 'disk-raid5'  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.59GiB path /dev/sdc
        devid    4 size 6.00GiB used 2.59GiB path /dev/sdd
        devid    5 size 6.00GiB used 2.56GiB path /dev/sde
        *** Some devices missing

It shows missing devid 3. Which wasn’t sdf. But sdd. This is what is false reported. Seems like the /dev/ assignments got shuffled somehow.

So let’s see if we can repair this. Let’s try mounting it.

sudo mount -v -t btrfs LABEL=disk-raid5 /media/btrfs-raid5/

Won’t work. It seems we need a ‘-o degraded’ status.

sudo mount -v -t btrfs -o degraded LABEL=disk-raid5 /media/btrfs-raid5/

This should work.

df -h
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/btrfs--vg-root  4.6G  1.9G  2.5G  44% /
none                        4.0K     0  4.0K   0% /sys/fs/cgroup
udev                        477M   12K  477M   1% /dev
tmpfs                        98M  1.2M   97M   2% /run
none                        5.0M     0  5.0M   0% /run/lock
none                        488M     0  488M   0% /run/shm
none                        100M     0  100M   0% /run/user
/dev/sda1                   236M  100M  124M  45% /boot
/dev/sdc                     24G  5.3G   13G  31% /media/btrfs-raid5
sudo btrfs fi show

When we inspect the file system, we can see the devid 3 isn’t mapped back in. However it still ‘knows’ what amount of data there should be.

Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    3 size 6.00GiB used 2.56GiB path
        devid    4 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    5 size 6.00GiB used 2.56GiB path /dev/sde

Let’s verify if it has some impact on the data.

md5sum /media/btrfs-raid5/*
03486548bc7b0f1a3881dc00c0f8c5f8  /media/btrfs-raid5/S01E01 FLEMISH HDTV x264.mp4
a9390aed84a6be8c145046772296db26  /media/btrfs-raid5/S01E02 FLEMISH HDTV x264.mp4
2e37ed514579ac282986efd78ac3bb76  /media/btrfs-raid5/S01E03 FLEMISH HDTV x264.mp4
1596a5e56f14c843b5c27e2d3ff27ebd  /media/btrfs-raid5/S01E04 FLEMISH HDTV x264.mp4
f7d494d6858391ac5c312d141d9ee0e5  /media/btrfs-raid5/S01E05 FLEMISH HDTV x264.mp4
fe6f097ff136428bfc3e2a1b8e420e4e  /media/btrfs-raid5/S01E06 FLEMISH HDTV x264.mp4
43c5314079f08570f6bb24b5d6fde101  /media/btrfs-raid5/S01E07 FLEMISH HDTV x264.mp4
3b5ea952b632bbc58f608d64667cd2a1  /media/btrfs-raid5/S01E08 FLEMISH HDTV x264.mp4
db6b8bf608de2008455b462e76b0c1dd  /media/btrfs-raid5/S01E09 FLEMISH HDTV x264.mp4
0d5775373e1168feeef99889a1d8fe0a  /media/btrfs-raid5/S01E10 FLEMISH HDTV x264.mp4
8dd4b25c249778f197fdb33604fdb998  /media/btrfs-raid5/S01E11 FLEMISH HDTV x264.mp4
edac6a857b137136a4d27bf6926e1287  /media/btrfs-raid5/S01E12 FLEMISH HDTV x264.mp4

Still all good. Perfomance seems to be severly impacted, however this will be due to the missing drive.

Now delete all missing disks from the file system.

sudo btrfs device delete missing /media/btrfs-raid5/

As you can see, this causes a rebalance. So a full disk will likely fail at this point. And a replace should be used instead this approach.

sudo btrfs fi show
        Total devices 3 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.88GiB path /dev/sdc
        devid    4 size 6.00GiB used 2.88GiB path /dev/sdd
        devid    5 size 6.00GiB used 2.88GiB path /dev/sde

Let’s reuse the /dev/sdb, so wipe it first.

sudo wipefs /dev/sdb

Now add this disk to the btrfs RAID5.

sudo btrfs device add /dev/sdb /media/btrfs-raid5
sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.88GiB path /dev/sdc
        devid    4 size 6.00GiB used 2.88GiB path /dev/sdd
        devid    5 size 6.00GiB used 2.88GiB path /dev/sde
        devid    6 size 6.00GiB used 0.00 path /dev/sdb

Now that we have added the /dev/sdb disk we can see a severe imbalance in the files. Luckily this can be easily fixed with a balance.

sudo btrfs balance start /media/btrfs-raid5
sudo btrfs fi show
Label: disk-raid5  uuid: 5e8d29ae-aea8-4460-a049-fae62e9994fd
        Total devices 4 FS bytes used 5.28GiB
        devid    2 size 6.00GiB used 2.56GiB path /dev/sdc
        devid    4 size 6.00GiB used 2.56GiB path /dev/sdd
        devid    5 size 6.00GiB used 2.56GiB path /dev/sde
        devid    6 size 6.00GiB used 2.56GiB path /dev/sdb
</p
Let's check our files again
<pre lang="Bash">
md5sum /media/btrfs-raid5/*
03486548bc7b0f1a3881dc00c0f8c5f8  /media/btrfs-raid5/S01E01 FLEMISH HDTV x264.mp4
a9390aed84a6be8c145046772296db26  /media/btrfs-raid5/S01E02 FLEMISH HDTV x264.mp4
2e37ed514579ac282986efd78ac3bb76  /media/btrfs-raid5/S01E03 FLEMISH HDTV x264.mp4
1596a5e56f14c843b5c27e2d3ff27ebd  /media/btrfs-raid5/S01E04 FLEMISH HDTV x264.mp4
f7d494d6858391ac5c312d141d9ee0e5  /media/btrfs-raid5/S01E05 FLEMISH HDTV x264.mp4
fe6f097ff136428bfc3e2a1b8e420e4e  /media/btrfs-raid5/S01E06 FLEMISH HDTV x264.mp4
43c5314079f08570f6bb24b5d6fde101  /media/btrfs-raid5/S01E07 FLEMISH HDTV x264.mp4
3b5ea952b632bbc58f608d64667cd2a1  /media/btrfs-raid5/S01E08 FLEMISH HDTV x264.mp4
db6b8bf608de2008455b462e76b0c1dd  /media/btrfs-raid5/S01E09 FLEMISH HDTV x264.mp4
0d5775373e1168feeef99889a1d8fe0a  /media/btrfs-raid5/S01E10 FLEMISH HDTV x264.mp4
8dd4b25c249778f197fdb33604fdb998  /media/btrfs-raid5/S01E11 FLEMISH HDTV x264.mp4
edac6a857b137136a4d27bf6926e1287  /media/btrfs-raid5/S01E12 FLEMISH HDTV x264.mp4

To verify if the BTRFS keeps existing you can reboot.

sudo reboot

Test Five: Byte corruption.

For this test I will fill the entire RAID with as much data as possible. (6 drives should equal to around 18 GB of data)

du -sh /media/btrfs-raid5
18G     /media/btrfs-raid5
btrfs filesystem df /media/btrfs-raid5/
Data, RAID5: total=17.62GiB, used=17.02GiB
System, RAID5: total=96.00MiB, used=16.00KiB
Metadata, RAID5: total=288.00MiB, used=19.20MiB
unknown, single: total=16.00MiB, used=0.00

Also note that df isn’t really a great tool to calculate size free for our BTRFS parition.

df -h
Filesystem                  Size  Used Avail Use% Mounted on
/dev/mapper/btrfs--vg-root  4.6G  1.9G  2.5G  44% /
none                        4.0K     0  4.0K   0% /sys/fs/cgroup
udev                        477M  4.0K  477M   1% /dev
tmpfs                        98M  1.3M   97M   2% /run
none                        5.0M     0  5.0M   0% /run/lock
none                        488M     0  488M   0% /run/shm
none                        100M     0  100M   0% /run/user
/dev/sda1                   236M  100M  124M  45% /boot
/dev/sdb                     24G   18G   50M 100% /media/btrfs-raid5

Now to test this I will shutdown the machine and use wxHexEditor to corrupt some bytes of our virtual disks. This emulates a disk writing bad bytes.

So once this is done start the machine again.

cat /var/log/syslog | grep BTRFS

This shows no errors, which is normal as BTRFS hasn’t started scrubbing. To trigger a scrub, you can access the file (which will trigger the checksum). Or we can start a scrub of the disks manually.

sudo btrfs scrub start /media/btrfs-raid5/

Once scrubbing starts we can follow the process.

sudo watch btrfs scrub status /media/btrfs-raid5/
scrub status for 5e8d29ae-aea8-4460-a049-fae62e9994fd
        scrub started at Sun Nov 22 17:08:46 2015, running for 120 seconds
        total bytes scrubbed: 15.62GiB with 3 errors
        error details: csum=3
        corrected errors: 3, uncorrectable errors: 0, unverified errors: 0

Now syslog will show errors popping up.

Nov 22 17:09:22 btrfs kernel: [  261.969305] BTRFS: checksum error at logical 44498075648 on dev /dev/sdc, sector 10316768, root 5, inode 281, offset 569163776, length 4096, links 1 (path: S01E05.mkv)
Nov 22 17:09:22 btrfs kernel: [  261.969310] BTRFS: bdev /dev/sdc errs: wr 0, rd 0, flush 0, corrupt 1, gen 0
Nov 22 17:09:22 btrfs kernel: [  262.159200] BTRFS: fixed up error at logical 44498075648 on dev /dev/sdc
Nov 22 17:09:28 btrfs kernel: [  267.507804] BTRFS: checksum error at logical 48935047168 on dev /dev/sdc, sector 12507592, root 5, inode 287, offset 426938368, length 4096, links 1 (path: S02E03.mkv)
Nov 22 17:09:28 btrfs kernel: [  267.507809] BTRFS: bdev /dev/sdc errs: wr 0, rd 0, flush 0, corrupt 2, gen 0
Nov 22 17:09:28 btrfs kernel: [  267.717962] BTRFS: fixed up error at logical 48935047168 on dev /dev/sdc
Nov 22 17:10:29 btrfs kernel: [  328.740414] BTRFS: checksum error at logical 45808136192 on dev /dev/sdc, sector 11169624, root 5, inode 283, offset 555790336, length 4096, links 1 (path: S01E07.mkv)

So this were the lengthy RAID5 tests and the last part of BTRFS. Next up is mhddfs.

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