[Gluster-devel] dht: selfheal of missing directories on nameless (by GFID) LOOKUP

[Anand Avati]

On Sun, May 4, 2014 at 9:22 AM, Niels de Vos <ndevos at redhat.com> wrote:

> Hi,
>
> bug 1093324 has been opened and we have identified the following cause:
>
> 1. an NFS-client does a LOOKUP of a directory on a volume
> 2. the NFS-client receives a filehandle (contains volume-id + GFID)
> 3. add-brick is executed, but the new brick does not have any
>    directories yet
> 4. the NFS-client creates a new file in the directory, this request is
>    in the format or <filehandle>/<filename>, <filehandle> was received
>    in step 2
> 5. the NFS-server does a LOOKUP on the parent directory identified by
>    the filehandle - nameless LOOKUP, only GFID is known
> 6. the old brick(s) return successfully
> 7. the new brick returns ESTALE
> 8. the NFS-server returns ESTALE to the NFS-client
>
> In this case, the NFS-client should not receive an ESTALE. There is also
> no ESTALE error passed to the client when this procedure is done over
> FUSE or samba/libgfapi.
>
> Selfhealing a directory entry based only on a GFID is not always
> possible. Files do not have a unique filename (hardlinks), so it is not
> trivial to find a filename for a GFID (expensive operation, and the
> result could be a list). However, for a directory this is simpler.
> A directory is not hardlink'd in the .glusterfs directory, directories
> are maintained as symbolic-links. This makes it possible to find the
> name of a directory, when only the GFID is known.
>
> Currently DHT is not able to selfheal directories on a nameless LOOKUP.
> I think that it should be possible to change this, and to fix the ESTALE
> returned by the NFS-server.
>
> At least two changes would be needed, and this is where I would like to
> hear opinions from others about it:
>
> - The posix-xlator should be able to return the directory name when
>   a GFID is given. This can be part of the LOOKUP-reply (dict), and that
>   would add a readlink() syscall for each nameless LOOKUP that finds
>   a directory. Or (suggested by Pranith) add a virtual xattr and handle
>   this specific request with an additional FGETXATTR call.
>

I think the LOOKUP-reply with readlink() is better, instead of a new
over-the-wire FOP.


>
> - DHT should selfheal the directory when at least one ESTALE is returned
>   by the bricks.



This also makes sense, except - if even the parent directory is missing on
that server (yet to be healed). Another important point to note is that,
the directories (with the same GFID) themselves may be present at various
locations as various dentries on the many servers. A lookup of
<dir-gfid>/"name" should succeed transparently independent of the differing
<dir-gfid>'s dentries across servers.

However if you want to heal, now the choice of server from where you select
the dir's parent and name become important as the self-heal will impose
that on the other servers. For e.g one of the AFR subvolumes may have not
yet healed the parent directories etc. Or, the N-1 servers may each return
a different par-gfid/dir-name in the LOOKUP reply. So it can quickly get
hairy.

As a general approach, using the LOOKUP-reply to send parent info from the
posix level makes sense. But we also need a more detailed proposal on how
that info is used at the cluster xlator levels to achieve a higher level
goal, like self-heal.


> When all bricks return ESTALE, the ESTALE is valid and
>   should be passed on to the upper layers (NFS-server -> NFS-client).
>

Yes.

Thanks
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