On 08/09/2014 17:09, David Sterba wrote:
On Sat, Sep 06, 2014 at 04:05:20PM -0400, Chris Mason wrote:
On 09/03/2014 09:36 AM, Miao Xie wrote:
This patchset implements device list automatic building function. As we
know, currently we need scan the devices to build device list by a user tool
before mounting the filesystem, especially mount the filesystem after
we re-install btrfs module. It is not convenient. This patchset can improve
that problem. With this patchset, we will scan all the devices in the
system to build the device list if we find the number of the devices
is not right when we mount the filesystem. By this way, we needn't scan
the device by the user tool and reduce the mount failure probability due
to the incomplete device list.
Thanks for working on these patches, but I really prefer that we do
these scans from userspace.
I agree. The userspace approach gives more control of when and how the
scanning is done. Yes the scan has to be done after the module is
inserted, but distros have that in initrd. Reinstalling module is not
something a normal user does and developers know how to workaround.
Automatic scanning in usperspace can be done via the mount helper and
this is IMO the way to go. There's a patch for that from Goffredo, not
merged yet, the patch backlog is still too long.
Since we are on the topic of scanning. A point on improving the
btrfs-progs scan method which I am working on...
- Scan of all system devices is an expensive task. But btrfs-progs
do it very liberally. Due to this there are some serious problem
like - btrfs fi show is too slow when scrub is running.
- The worst is Single btrfs-progs thread scans for btrfs
devices multiple times. Mainly because most of the functions
uses check_mounted() which in turn calls scan, think of
multi device btrfs config.
- The problem would be more prominent in larger server with 1000's
of LUNs / devices.
- lblkid can be the only method to scan for devices. (The other
method we have as of now is of canning /proc/partitions, which
lblkid also does).
what I am planning -
Use btrfs control ioctl to help btrfs-progs check_mounted() to know
if a (multi) device is mounted.
Build kernel's fs_devices list in the user-space using btrfs
control ioctl.
Do device scan only once per btrfs-progs thread.
Any comments ?
Thanks, Anand
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