Re: [PATCH v2 2/2] btrfs: sysfs add devinfo/fsid to retrieve fsid from the device

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

On 17/11/2021 01:16, David Sterba wrote:
> On Thu, Oct 21, 2021 at 11:31:17PM +0800, Anand Jain wrote:
>> In the case of the seed device, the fsid can be different from the mounted
>> sprout fsid.  The userland has to read the device superblock to know the
>> fsid but, that idea fails if the device is missing. So add a sysfs
>> interface devinfo/<devid>/fsid to show the fsid of the device.
>>
>> For example:
>>   $ cd /sys/fs/btrfs/b10b02a5-f9de-4276-b9e8-2bfd09a578a8
>>
>>   $ cat devinfo/1/fsid
>>   c44d771f-639d-4df3-99ec-5bc7ad2af93b
>>   $ cat  devinfo/3/fsid
>>   b10b02a5-f9de-4276-b9e8-2bfd09a578a8
> 
>  From user perspective, it's another fsid, one is in the path, so I'm
> wondering if it should be named like read_fsid or sprout_fsid or if the
> seed/sprout information should be put into another directory completely.

I am viewing it as fsid as per the device's sb. This fsid matches with
the blkid(8) output. A path to the device's fsid will help to script.
So I am not voting for sprout_fsid because it does not exist in the
most common non-sprouted fsid.

Now to show whether a device is seed or sprout, the user friendly
approach will be like for example:
      /sys/fs/btrfs/<fsid>/devinfo/<devid>/device_state
to show all that apply, for example:
      SEED|SPROUT|READ_ONLY|WRITABLE|ZONED|METADATA_ONLY|\
      DATA_ONLY|READ_PREFERRED|REPLACE_TGT|REPLACE_SRC|\
      SCRUB_RUNNING

However, per kernel general rule of thumb one value per sys-fs file,
so it should be for example:
      /sys/fs/btrfs/<fsid>/devinfo/<devid>/seed
0
      /sys/fs/btrfs/<fsid>/devinfo/<devid>/sprout
1

So based on this my patch "btrfs: introduce new device-state
read_preferred" in the mailing list already proposed
      /sys/fs/btrfs/<fsid>/devinfo/<devid>/read_preferred

So to summarize,  I can rename fsid to read_fsid. And, progressively
in a separate patch, we can add <>/seed <>/sprout sysfs files as above.
What do you think?