How to fill in the fsid for file systems?

[Rick Macklem]

Andriy Gapon wrote:
>On 04/11/2018 17:38, Rick Macklem wrote:
>> That leaves ZFS, which is what I was asking.
>> I don't understand the ZFS code well enough to understand  what
>> dmu_objset_fsid_guid() is doing to get the fsid. I also don't know if the value
>> changes for snaphots of the same file system?
>> - If it does ever change, then that is the usage case for this option.
>
>A good ZFS example is this.
>A filesystem is duplicated using zfs send + zfs recv.  The content would be
>exactly the same (provided no modification of the source and the target after
>the duplication), but GUIDs of the source and target would be different.  The
>latter is required because the source and the target could be in the same pool.
>But even if they are in different pools (and different machines), the GUIDs
>would still be different.
Cool. Assuming the duplicated file system created via zfs send + zfs recv still
has the same fid for any given file (the stuff used by zfs_fid() stays the same
for any given file in the file system), then this would be good use case.

It will need some ZFS magic to make a generic fs mount option work for ZFS.
Maybe you can do that?

The current patch I have for adding a generic "fsid=N" mount option is in
D17839. I've also attached it to this message in case that is easier.
(It currently doesn't allow fsid=N to be used during an update "mount -u".
 This made sense to me, since an fsid shouldn't change, but if others think
  it should be changeable via "mount -u" the patch can easily be changed.)

rick
ps: I did notice the code is "slightly racy" in that there is a period of time between
      when VFS_MOUNT() sets the mnt_stat.f_fsid and when the mount structure is 
      entered in the mountlist,  where another mount could happen using the same
      mnt_stat.f_fsid value, but that has been there for a long time and wouldn't be
      easy to fix.

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