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--- linux:filesystems:brtfs [2017/01/09 00:46]
tkilla [debug / recovery]
+++ linux:filesystems:brtfs [2025/01/08 23:53] (current)
tkilla [debug / recovery]
@@ Line 9: / Line 9: @@
 **install 4.0 from jessie backports** https://backports.debian.org/Instructions/
+===== important notes =====
+btrfs is really tricky and needs a lot of maintaince:
+  * balance weekly
+  * use btrfs fi usage /home/ to see disc space! btrfs fi show /home/ shows *allocated* space, which is always almost everything
+  * leave 20% free :-0
+  * disc full can cause various crashes: even mysql may suffer and kill a connected slave db & so also php processes! (it seems)
+  * Use the new usage command, it has more information than btrfs fi show and btrfs fi df - see below
@@ Line 68: / Line 80: @@
 altough df shows free space :-0
-only this shows the real free space:
+Detailed description of the meanings of the command outputs: http://ram.kossboss.com/btrfs-fi-df-balance-btrfs-df-and-freespace-req/
+**Use the new usage command, it has more information than btrfs fi show and btrfs fi df**
+  btrfs fi df /home/
+this is quite confusing:
+total shows the allocated space, not the space. Used shows the data-usage space. you need to add metadata usage.
   btrfs fi df /home/
@@ Line 76: / Line 96: @@
   btrfs filesystem show
+for example: here only 2.67TiB are *really* used, but because of shitty block usage 3.28TiB are full!! and only ~23GB free. balance required!
+  Total devices 2 FS bytes used 2.67TiB
+  devid    1 size 3.51TiB used 3.28TiB path /dev/sda4
+  devid    2 size 3.51TiB used 3.28TiB path /dev/sdb4
 if the device was full, reorder chunks - kind of defrag:
-  btrfs fi balance start -dusage=5 /home/
+  btrfs fi balance start -v -dusage=5 /home/
+increase dusage value up to 95
@@ Line 86: / Line 116: @@
   * https://coreos.com/os/docs/latest/btrfs-troubleshooting.html
+  * http://marc.merlins.org/perso/btrfs/post_2014-05-04_Fixing-Btrfs-Filesystem-Full-Problems.html
+===== btrfs RAID =====
+**If you run a RAID1 and one device crashs, you have a problem!  The device cannot be removed: Cannot go below the minimum number of raid devices (2)**
+Another problem is, that **a degraded RAID can only be mounted RW ONCE!** if you can only mount it RO, you cannot add/remove/replace a device.
+The solution is to replace failed device, **after the failed disc has been replaced pysically.**
+Quote from the wiki:
+In case of raidXX layout, you cannot go below the minimum number of the device required. So before removing a device (even the missing one) you may need to add a new one. For example if you have a raid1 layout with two device, and a device fails, you must:
+. partition the disc. use sgdisk to copy partition layout:
+    sgdisk --replicate=/dev/$dest $source
+    sgdisk --randomize-guids /dev/$dest  # NEW UUIDS!
+    partprobe /dev/sda* /dev/sdb*  #..tell the kernel: partprobe /dev/sd*
+Fix mdadm for /boot and swap partis: https://wiki.fr33.info/doku.php/linux/filesystems/raid?s[]=mdadm
+Fix btrfs:
+    mount btrfs in degraded mode # to get rw ("degraded" in /etc/default/grub) should mount / degraded
+    (add a new device, remove the missing device)  # old, better replace
+    btrfs replace start /dev/sdaX /dev/sdbX /
+Useful articles:
+  * http://blog.programster.org/recover-btrfs-array-after-device-failure/
+  * https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Replacing_failed_devices
 ===== debug /  recovery =====
+==== Danger Zone !!:!: ====
+**These commands may destroy filesystems!1!**
+There may be problems, if btrfs runs on top of mdadm raid..maybe.. or maybe the reason is:
 **balance errors:**
@@ Line 98: / Line 170: @@
 balance is also restarted after reboot and the system hangs again!
 mount without balance, maybe disable fstab line and hard reboot before
   mount -o recovery,skip_balance /dev/sdaX /mnt/xx
+**be patient: this can take 1-2 hours!**
 after remounting with skip_balance, you can cancel the balance
-  btrfs balance cancel /mn/xx
+  btrfs balance cancel /mnt/xx
@@ Line 111: / Line 185: @@
   mount -o recovery /dev/sdaX /mnt/xx
+** recover a partition, that cannot be mounted**
+errors like these:
+  parent transid verify failed on 31302336512 wanted 62455 found 62456
+  Remounting read-write after error is not allowed
+  can't read superblock
+Try this:
+  mount -t btrfs -o nospace_cache /dev/mdX /home/
+This will be very slow, but might work.
+Maybe you need to reboot first.
+Try this otherwise:
+  btrfs rescue zero-log /dev/md4
@@ Line 125: / Line 219: @@
   * https://seravo.fi/2015/using-raid-btrfs-recovering-broken-disks
   * https://btrfs.wiki.kernel.org/index.php/Problem_FAQ
+===== links =====
+  * convert mdadm raid to btrfs raid: https://tech.feedyourhead.at/content/btrfs-turning-mdadm-array-btrfs

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