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

[Goffredo Baroncelli <kreijack@inwind.it>]

On 09/08/2014 01:04 PM, Anand Jain wrote:
> 
> 
> 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.

Using libblkid (see your proposal below) would help in this case, 
because the values would be cached.
> 
>    - 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.

You are talking about a ioctl: why not use (extending them when needed) 
the sysfs information ?

> 
> Any comments ?


> 
> Thanks, Anand
> 
> 
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