Re: [PATCH RFC 0/5] Scan all devices to build fs device list

[Anand Jain <Anand.Jain@oracle.com>]

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