Re: inode linking in GlusterFS NFS server

[Raghavendra Bhat <rabhat@xxxxxxxxxx>]

On Tuesday 08 July 2014 01:21 AM, Anand Avati wrote:

On Mon, Jul 7, 2014 at 12:48 PM, Raghavendra Bhat <rabhat@xxxxxxxxxx> wrote:

Hi,

As per my understanding nfs server is not doing inode linking in readdirp callback. Because of this there might be some errors while dealing with virtual inodes (or gfids). As of now meta, gfid-access and snapview-server (used for user serviceable snapshots) xlators makes use of virtual inodes with random gfids. The situation is this:

Say User serviceable snapshot feature has been enabled and there are 2 snapshots ("snap1" and "snap2"). Let /mnt/nfs be the nfs mount. Now the snapshots can be accessed by entering .snaps directory.  Now if snap1 directory is entered and *ls -l* is done (i.e. "cd /mnt/nfs/.snaps/snap1" and then "ls -l"),  the readdirp fop is sent to the snapview-server xlator (which is part of a daemon running for the volume), which talks to the corresponding snapshot volume and gets the dentry list. Before unwinding it would have generated random gfids for those dentries.

Now nfs server upon getting readdirp reply, will associate the gfid with the filehandle created for the entry. But without linking the inode, it would send the readdirp reply back to nfs client. Now next time when nfs client makes some operation on one of those filehandles, nfs server tries to resolve it by finding the inode for the gfid present in the filehandle. But since the inode was not linked in readdirp, inode_find operation fails and it tries to do a hard resolution by sending the lookup operation on that gfid to the normal main graph. (The information on whether the call should be sent to main graph or snapview-server would be present in the inode context. But here the lookup has come on a gfid with a newly created inode where the context is not there yet. So the call would be sent to the main graph itself). But since the gfid is a randomly generated virtual gfid (not present on disk), the lookup operation fails giving error.

As per my understanding this can happen with any xlator that deals with virtual inodes (by generating random gfids).

I can think of these 2 methods to handle this:
1)  do inode linking for readdirp also in nfs server
2)  If lookup operation fails, snapview-client xlator (which actually redirects the fops on snapshot world to snapview-server by looking into the inode context) should check if the failed lookup is a nameless lookup. If so, AND the gfid of the inode is NULL AND lookup has come from main graph, then instead of unwinding the lookup with failure, send it to snapview-server which might be able to find the inode for the gfid (as the gfid was generated by itself, it should be able to find the inode for that gfid unless and until it has been purged from the inode table).


Please let me know if I have missed anything. Please provide feedback.

That's right. NFS server should be linking readdirp_cbk inodes just like FUSE or protocol/server. It has been OK without virtual gfids thus far.

I did the changes to link inodes in readdirp_cbk in nfs server. It seems to work fine. Should we need the second change also? (i.e chage in the snapview-client to redirect the fresh nameless lookups to snapview-server). With nfs server linking the inodes in readdirp, I think second change might not be needed.

Regards,
Raghavendra Bhat

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