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

[Joe Julian]

If the time is set on a file by the client, this increases the critical 
complexity to include the clients whereas before it was only critical to 
have the servers time synced, now the clients should be as well.

Just spitballing here, but what if the time was converted at the posix 
layer as a difference between the current time and the file time and 
converted back somewhere in the client graph? Each server's file time 
would differ by the same amount to its current time [1] so it should be 
a consistent value between servers.


[1] depending on drift, but if the admin can't manage clocks, there's 
not much gluster could or should do about that.

On 01/26/2016 12:07 AM, Joseph Fernandes wrote:
> 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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