Re: snapshot restore and USS

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On Monday 01 December 2014 05:36 PM, Raghavendra Bhat wrote:
On Monday 01 December 2014 04:51 PM, Raghavendra G wrote:


On Fri, Nov 28, 2014 at 6:48 PM, RAGHAVENDRA TALUR <raghavendra.talur@xxxxxxxxx> wrote:
On Thu, Nov 27, 2014 at 2:59 PM, Raghavendra Bhat <rabhat@xxxxxxxxxx> wrote:
> Hi,
>
> With USS to access snapshots, we depend on last snapshot of the volume (or
> the latest snapshot) to resolve some issues.
> Ex:
> Say there is a directory called "dir" within the root of the volume and USS
> is enabled. Now when .snaps is accessed from "dir" (i.e. /dir/.snaps), first
> a lookup is sent on /dir which snapview-client xlator passes onto the normal
> graph till posix xlator of the brick. Next the lookup comes on /dir/.snaps.
> snapview-client xlator now redirects this call to the snap daemon (since
> .snaps is a virtual directory to access the snapshots). The lookup comes to
> snap daemon with parent gfid set to the gfid of "/dir" and the basename
> being set to ".snaps". Snap daemon will first try to resolve the parent gfid
> by trying to find the inode for that gfid. But since that gfid was not
> looked up before in the snap daemon, it will not be able to find the inode.
> So now to resolve it, snap daemon depends upon the latest snapshot. i.e. it
> tries to look up the gfid of /dir in the latest snapshot and if it can get
> the gfid, then lookup on /dir/.snaps is also successful.

From the user point of view, I would like to be able to enter into the
.snaps anywhere.
To be able to do that, we can turn the dependency upside down, instead
of listing all
snaps in the .snaps dir, lets just show whatever snapshots had that dir.

Currently readdir in snap-view server is listing _all_ the snapshots. However if you try to do "ls" on a snapshot which doesn't contain this directory (say dir/.snaps/snap3), I think it returns ESTALE/ENOENT. So, to get what you've explained above, readdir(p) should filter out those snapshots which doesn't contain this directory (to do that, it has to lookup dir on each of the snapshots).

Raghavendra Bhat explained the problem and also a possible solution to me in person. There are some pieces missing in the problem description as explained in the mail (but not in the discussion we had). The problem explained here occurs  when you restore a snapshot (say snap3) where the directory got created, but deleted before next snapshot. So, directory doesn't exist in snap2 and snap4, but exists only in snap3. Now, when you restore snap3, "ls" on dir/.snaps should show nothing. Now, what should be result of lookup (gfid-of-dir, ".snaps") should be?

1. we can blindly return a virtual inode, assuming there is atleast one snapshot contains dir. If fops come on specific snapshots (eg., dir/.snaps/snap4), they'll anyways fail with ENOENT (since dir is not present on any snaps).
2. we can choose to return ENOENT if we figure out that dir is not present on any snaps.

The problem we are trying to solve here is how to achieve 2. One simple solution is to lookup for <gfid-of-dir> on all the snapshots and if every lookup fails with ENOENT, we can return ENOENT. The other solution is to just lookup in snapshots before and after (if both are present, otherwise just in latest snapshot). If both fail, then we can be sure that no snapshots contain that directory.

Rabhat, Correct me if I've missed out anything :).



If a readdir on .snaps entered from a non root directory has to show the list of only those snapshots where the directory (or rather gfid of the directory) is present, then the way to achieve will be bit costly.

When readdir comes on .snaps entered from a non root directory (say ls /dir/.snaps), following operations have to be performed
1) In a array we have the names of all the snapshots. So, do a nameless lookup on the gfid of /dir on all the snapshots
2) Based on which snapshots have sent success to the above lookup, build a new array or list of snapshots.
3) Then send the above new list as the readdir entries.

But the above operation it costlier. Because, just to serve one readdir request we have to make a lookup on each snapshot (if there are 256 snapshots, then we have to make 256 lookup calls via network).

One more thing is resource usage. As of now any snapshot will be initied (i.e. via gfapi a connection is established with the corresponding snapshot volume, which is equivalent to a mounted volume.) when that snapshot is accessed (from fops point of view a lookup comes on the snapshot entry, say "ls /dir/.snaps/snap1").  Now to serve readdir all the snapshots will be  accessed and all the snapshots are initialized. This means there can be 256 instances of gfapi connections with each instance having its own inode table and other resources). After readdir if a snapshot is not accessed, so many resources of that snapshots will add up to the snap daemon's usage.

From a NFS mount moint, if we do 'ls /dir/.snaps/', NFS client will send a stat on all the entries and only after this it returns to the application. This will also initiate gfapi for all the snapshots right?

Thanks,
Vijay

With the above points in mind, I was thinking about different approaches to handle this situation. We need latest snapshot (and as per the patch, adjacent snapshots to handle restore) to resolve lookups coming on .snaps. Mainly for resolving the parent gfid so that we can look it up somewhere (if "ls /dir/.snaps is done, then lookup comes with parent gfid set to gfid of /dir and name set to ".snaps". But since /dir has not been looked up yet in snap daemon, it has to first resolve parent gfid for which it looks at latest snapshot).

What we can do is, while sending lookup on .snaps (again, say "ls /dir/.snaps") within the dict add a key, which snapview-server can look for. That key is kinda hint from snapview-client to the snapview-server that the parent gfid of this particular lookup call exists and valid one. When snapview-server gets lookup as part of resolution from protocol/server on the parent gfid, it can look at the dict for the key. If the key is set, then simply return success to that lookup.

With the above way we can handle many situations such as this:
Entering .snaps from a directory which is created after taking the latest snapshot.

Please provide feedback on the above approach (the hint being set in the dict).

Regards,
Raghavendra Bhat





May be it is good enough if we resolve the parent on the main volume
and rely on that
in snapview client and server.

>
> But, there can be some confusion in the case of snapshot restore. Say there
> are 5 snapshots (snap1, snap2, snap3, snap4, snap5) for a volume vol. Now
> say the volume is restored to snap3. If there was a directory called
> "/a" at the time of taking snap3 and was later removed, then after snapshot
> restore accessing .snaps from that directory (in fact all the directories
> which were present while taking snap3) might cause problems. Because now the
> original volume is nothing but the snap3 and snap daemon when gets the
> lookup on "/a/.snaps", it tries to find the gfid of "/a" in the latest
> snapshot (which is snap5) and if a was removed after taking snap3, then the
> lookup of "/a" in snap5 fails and thus the lookup of "/a/.snaps" will also
> fail.


>
> Possible Solution:
> One of the possible solution that can be helpful in this case is, whenever
> glusterd sends the list of snapshots to snap daemon after snapshot restore,
> send the list in such a way that the snapshot which is previous to the
> restored snapshot is sent as the latest snapshot (in the example above,
> since snap3 is restored, glusterd should send snap2 as the latest snapshot
> to snap daemon).
>
> But in the above solution also, there is a problem. If there are only 2
> snapshots (snap1, snap2) and the volume is restored to the first snapshot
> (snap1), there is no previous snapshot to look at. And glusterd will send
> only one name in the list which is snap2 but it is in a future state than
> the volume.
>
> A patch has been submitted for the review to handle this
> (http://review.gluster.org/#/c/9094/).
> And in the patch because of the above confusions snapd tries to consult the
> adjacent snapshots  of the restored snapshot to resolve the gfids. As per
> the 5 snapshots example, it tries to look at snap2 and snap4 (i.e. look into
> snap2 first, if it fails then look into snap4). If there is no previous
> snapshot, then look at the next snapshot (2 snapshots example). If there is
> no next snapshot, then look at the previous snapshot.
>
> Please provide feed back about how this issue can be handled.
>
> Regards,
> Raghavendra Bhat
> _______________________________________________
> Gluster-devel mailing list
> Gluster-devel@xxxxxxxxxxx
> http://supercolony.gluster.org/mailman/listinfo/gluster-devel



--
Raghavendra Talur
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--
Raghavendra G



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