btrfs device scan should be performed before fstab mounts

saulgoode · 10-06-2014, 05:18 AM

I have been playing with BTRFS on Slackware64 14.1 and encountered a situation that is apparently not addressed by the sysvinit scripts.

In short, my btrfs volumes do not get recognized until a 'btrfs device scan' is performed and since this is not performed by any of the /etc/rc.d init scripts, mounting of those volumes as specified in fstab fails. Adding the command to rc.S resolves the issue (I added it around line 50, directly after the UDEV initialization).

It is possible that this behavior occurs because my btrfs volumes were created directly on the devices (/dev/sdb and /dev/sdc) and not on partitions on the devices (e.g., /dev/sdb1). I have not explicitly tested this hypothesis but it seems earlier in my experimentation that things worked properly when btrfs volumes were mounted on partitions.

I have not checked on whether this issue arises in -current but I did check the sysvinit scripts and note that the scan command is not there; though it is possible that this issue has been addressed upstream.

ReaperX7 · 10-06-2014, 10:59 PM

BtrFS is still labeled as experimental in Slackware, but yes, handling BtrFS devices rather than just partitions should be addressed. Can you provide a copy of the rc.S file you edited as an example, or at least a .diff style patch that could be used?

saulgoode · 10-06-2014, 11:35 PM

The following patch was created with "diff -u5 rc.S rc.S.patched".

Code:

--- rc.S        2013-10-28 01:06:39.000000000 -0400
+++ rc.S.patched        2014-10-07 00:22:10.428005892 -0400
@@ -46,10 +46,13 @@
       /bin/sh /etc/rc.d/rc.udev start
     fi
   fi
 fi
 
+# Following is apparently needed by btrfs mounts appearing in /etc/fstab:
+btrfs device scan --all-devices
+
 # Mount Control Groups filesystem interface:
 if grep -wq cgroup /proc/filesystems ; then
   if [ -d /sys/fs/cgroup ]; then
     # See linux-*/Documentation/cgroups/cgroups.txt (section 1.6)
     # Check if we have some tools to autodetect the available cgroup controllers

I am not certain the scan is in the right location. I figured it should be after UDEV initialization in order to account for filesystems that might be on hotplugged external drives.

ReaperX7 · 10-07-2014, 01:30 AM

If it works at that point it should be useful at scanning btrfs volumes and partitions. I may incorporate this into my system as well.

GazL · 10-07-2014, 04:43 AM

Sticking it next to the lvm scan would seem the logical place to me as they both do the same sort of job.