btrfs, udev and btrfs

Hi all,

below a configuration for udev/initramfs which I propose to scan the block devices looking for a multi-volume btrfs filesystem.

Btrfs has the capability to span a file-system on multiple device. In order to do that, the involved devices have to be "registered" in the kernel.

In order to do that there are two options:

  # btrfs device scan <device>         (or the old tool btrfsctl -A <device>)

or

  # btrfs device scan          (or the old tool btrfsctl -a)

The first one scan the device <device> in order to find a btrfs volume. If this exists, it is registered in the kernel.

The second options scan all the block devices (even floppy, cdrom...) looking for a btrfs volume. This option has two disadvantage:
1) this commands is slow because it access also slow device
2) every time it scans all the block devices, even the ones already scanned.

Instead the first option has the disadvantage to need to be used for every new device.

excluding floppy and cdrom.

Below my udev rule

  $ cat /etc/udev/rules.d/60-btrfs.rules 

  # ghigo 15/04/2010

ACTION!="add|change", GOTO="btrfs_scan_end" SUBSYSTEM!="block", GOTO="btrfs_scan_end" KERNEL!="sd[!0-9]*|hd[!0-9]*", GOTO="btrfs_scan_end"

IMPORT{program}="/sbin/blkid -o udev -p $tempnode"

  # only disk scsi and ide
  ENV{ID_FS_TYPE}="btrfs", RUN+="/sbin/btrfsctl -A /dev/$name"

LABEL="btrfs_scan_end"

Note #1) This rule is tested on a ubuntu 10.4
Note #2) ubuntu 10.4 has the old btrfctl tool

This work quite well for devices which appear *after* mounting the root file system. For a (very) common internal hard disk, it is needed to update the initramfs. So I wrote other two scripts: an initramfs hook and a initramfs init-top scripts.

The initramfs init-top scripts, is ran during the boot: it load the btrfs module after all the other module.

  $ cat /etc/initramfs-tools/scripts/init-top/btrfs 
  #!/bin/sh

case $1 in

  # get pre-requisites
  prereqs)
        exit 0
        ;;
  esac

modprobe btrfs

The initramfs hook scripts, load the btrfsctl command, the btrfs module and the udev rule in the initramfs image:

  $ cat /etc/initramfs-tools/hooks/btrfs
  #!/bin/sh

case $1 in

  # get pre-requisites
  prereqs)
        echo "udev"
        exit 0
        ;;
  esac

. /usr/share/initramfs-tools/hook-functions

  mkdir -p $DESTDIR/lib/udev/rules.d/
  cp /etc/udev/rules.d/60-btrfs-scan.rules \
        $DESTDIR/lib/udev/rules.d/

copy_exec /sbin/btrfsctl
manual_add_modules btrfs
manual_add_modules crc32c

Note #1) pay attention to the fact the btrfs (libcrc32c in reality) needs the module crc32c even though there no is explicit dependency.
Note #2) init-top/btrfs has to start before init-top/udev: udev starts "btrfsctl", which requires the btrfs module. In my test the sequence is correct but I think that this is due to the fact that the scripts are ran in alphabetic order.
Note #3) when the module btrfs is loaded it creates the device /dev/btrfs- control; this device has to be created before the block devices. devtmpfs (which ubuntu 10.4 has) helps a lot in that.

CONCLUSION:
These scripts are a prototype for avoid to use explicitly the "btrfs device scan" command. To me it seems to work. I tested it with two usb disk which I inserted after the boot, and with two internal hard disks. Both contains a multi-device btrfs filesystem (but not a root filesystem).These scripts may be inserted in a package, thanks to the modularity of the debian initramfs tool.

NOTE: These scripts may eat your baby or drink your beer so used them carefully.
NOTE: I tested these scripts which an ubuntu 10.4 system. I don't know the changes which may be required to work for other distributions (like fedora), but I am very interested in knowing those.

Comments are welcome.

BR
Goffredo

gpg key@ keyserver.linux.it: Goffredo Baroncelli (ghigo) <kreijackATinwind.it>
Key fingerprint = 4769 7E51 5293 D36C 814E  C054 BF04 F161 3DC5 0512

Just few days ago I've asked dracut guys for including btrfs support.
Current result is here: http://dracut.git.sourceforge.net/git/gitweb.cgi?p=dracut/dracut;a=tree;f=modules.d/90btrfs
It still being polished a little.

Tomasz Torcz               RIP is irrevelant. Spoofing is futile.
xmpp: [email protected]     Your routes will be aggreggated. -- Alex Yuriev

every new

vices,

Udev needs to do this already anyway. People are not encouraged to call this in their own rule files again. Just make sure you place the rule after the existing standard call that always comes with udev. The btrfs rules can just depend on the variable set in the environment. Also there are more devices than sd* which could have a btrfs volume, but this is also covered by the standard udev call to blkid.

Thanks,
Kay
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