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

[Joseph Fernandes]

Answer inline:


----- Original Message -----
From: "Xavier Hernandez" <xhernandez at datalab.es>
To: "Pranith Kumar Karampuri" <pkarampu at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
Sent: Tuesday, January 26, 2016 1:21:37 PM
Subject: Re: [Gluster-devel] distributed files/directories and [cm]time	updates

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.

>>>>>>>>> JOE: the idea of metadata cache is cool for read work loads, but for writes we
would end up doing double writes to the disk. i.e 1 for the actual write or 1 to update the setxattr.
IMHO we cannot have it in a write back cache (periodic flush to disk) and ctime/mtime/atime data loss
or inconsistency will be a problem. Your thoughts?


Xavi
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