On 2013-11-03 22:23, Jeff Mahoney wrote:
On 11/3/13, 1:38 PM, Goffredo Baroncelli wrote:
Hi all,
this patch provides a mount.btrfs helper for the mount command to
mounting a btrfs filesystem.
A btrfs filesystem could spans several disks. This helper scans all
the partition to discover all the disks required to mount a filesystem.
So it is not necessary any-more to scan manually the partitions to mount
a filesystem via the btrfs device scan command.
It adds in the option parameters the devices required to mount a
filesystem. Supposing that a filesystem is composed by several disks
(/dev/sd[cdef]), when the user does mount /dev/sdd /mnt, mount calls
mount.btrfs which int turn calls the mount(2) syscall like:
mount(/dev/sdd, /mnt, btrfs, 0,
device=/dev/sdc,device=/dev/sde,device=/de/vsdf).
This helper uses both the libblkid and libmount to discover the
devices, to compute the parameters manipulation and to update the mtab
file.
I got the idea from the btrfs.wiki; its biggest gains is to avoid the
separation of scanning phases (at boot time or during the block device
discovery) from the mounting. Also mkfs.btrfs could avoid to re-do a
rescan of the devices after a formatting.
mount.btrfs doesn't add more requirement than the mount command. It
would be possible to remove the btrfs command from the initramfs, and
all the related scripts (in my debian both udev and btrfs-tools
packages contains three udev rules for btrfs).
Comments are welcome.
I like the idea but I'm concerned how well this will perform on large
systems with thousands of disks attached. btrfs dev scan can have a list
of potential devices specified, but this helper defaults to all devices.
liblkid uses a cache to increase the speed of a search.
Only if some incoherency from the data is found (like a mkfs after a
boot) a probe all is executed.
But in this case we could update mkfs.btrfs to invalidate the blkid
cache of disks involved (which should be done in any case)
A quick test run with 2000 scsi_debug targets shows that btrfs dev scan
can take 8 seconds, and that's with no actual I/O occurring. I don't
think we want to specify the list of devices to scan on the mount
command line, but perhaps an /etc config file that limits the scan to a
list of devices (and defaults to all) similar to how mdadm.conf works
would be a good improvement.
I tried in qemu-kvm:
# btrfs device scan # it took about 8sec
# modprobe scsi_debug num_tgts=2000 # 8 seconds were required
# before all udev instances ended
But when I performed a blkid -c /dev/null (which should rebuild the
cache) it takes ages
I have to investigate a bit...
-Jeff
BR
G.Baroncelli
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