[Gluster-devel] distributed files/directories and [cm]time updates

[Venky Shankar]

On Tue, Jan 26, 2016 at 08:51:37AM +0100, Xavier Hernandez wrote:
> Hi Pranith,
> 
> On 26/01/16 03:47, Pranith Kumar Karampuri wrote:
> >hi,
> >       Traditionally gluster has been using ctime/mtime of the
> >files/dirs on the bricks as stat output. Problem we are seeing with this
> >approach is that, software which depends on it gets confused when there
> >are differences in these times. Tar especially gives "file changed as we
> >read it" whenever it detects ctime differences when stat is served from
> >different bricks. The way we have been trying to solve it is to serve
> >the stat structures from same brick in afr, max-time in dht. But it
> >doesn't avoid the problem completely. Because there is no way to change
> >ctime at the moment(lutimes() only allows mtime, atime), there is little
> >we can do to make sure ctimes match after self-heals/xattr
> >updates/rebalance. I am wondering if anyone of you solved these problems
> >before, if yes how did you go about doing it? It seems like applications
> >which depend on this for backups get confused the same way. The only way
> >out I see it is to bring ctime to an xattr, but that will need more iops
> >and gluster has to keep updating it on quite a few fops.
> 
> I did think about this when I was writing ec at the beginning. The idea was
> that the point in time at which each fop is executed were controlled by the
> client by adding an special xattr to each regular fop. Of course this would
> require support inside the storage/posix xlator. At that time, adding the
> needed support to other xlators seemed too complex for me, so I decided to
> do something similar to afr.
> 
> Anyway, the idea was like this: for example, when a write fop needs to be
> sent, dht/afr/ec sets the current time in a special xattr, for example
> 'glusterfs.time'. It can be done in a way that if the time is already set by
> a higher xlator, it's not modified. This way DHT could set the time in fops
> involving multiple afr subvolumes. For other fops, would be afr who sets the
> time. It could also be set directly by the top most xlator (fuse), but that
> time could be incorrect because lower xlators could delay the fop execution
> and reorder it. This would need more thinking.
> 
> That xattr will be received by storage/posix. This xlator will determine
> what times need to be modified and will change them. In the case of a write,
> it can decide to modify mtime and, maybe, atime. For a mkdir or create, it
> will set the times of the new file/directory and also the mtime of the
> parent directory. It depends on the specific fop being processed.
> 
> mtime, atime and ctime (or even others) could be saved in a special posix
> xattr instead of relying on the file system attributes that cannot be
> modified (at least for ctime).
> 
> This solution doesn't require extra fops, So it seems quite clean to me. The
> additional I/O needed in posix could be minimized by implementing a metadata
> cache in storage/posix that would read all metadata on lookup and update it
> on disk only at regular intervals and/or on invalidation. All fops would
> read/write into the cache. This would even reduce the number of I/O we are
> currently doing for each fop.

That's exatly the route taken by DHTv2, although including other metadata such
as size, type, etc. For current gluster model, the above approach is clean.

However, with DHTv2, metadata is stored separately from data, in MDS (or rather
MDC: metadata cluster). The tricky part with MDS/DS split is to maintain filesystem
metadata consistency, especially object size after hard reboot of node(s).

Interested folks (in DHTv2 or generally) may take a look at this document (just a
initial draft):

       https://review.gerrithub.io/#/c/253517/

> 
> Xavi
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