[Gluster-devel] trusted.glusterfs.version xattr

[gordan at bobich.net]

On Wed, 7 May 2008, Krishna Srinivas wrote:

>>>> I suspect this isn't a problem that can be solved without having a
>>>> proper journal of metadata per directory, so that upon connection,
>>>> the whole journal can be replayed.
>>>>
>>>> You could sort of bodge it and use timestamps as the primary version and
>>>> the xattr version as secondary, bit that is no less dangerous - it
>>>> only takes one machine to be out of sync, and we are again looking at 
>>>> massive scope for data loss.
>>>>
>>>> You could bodge the bodge further to work around this by ensuring that
>>>> the nodes are heartbeating current times to sync between them and 
>>>> without the sync no data exchange takes place. But that then 
>>>> complicates things because what do you do when a node connects and is 
>>>> out of sync, but in the future? Who wins on time sync? Who has the 
>>>> latest authoritative copy?
>>>>
>>>> I think the most sane way of addressing this is to have a fully logged
>>>> directory metadata journal. But then we are back to the journalling 
>>>> for fast updates issue with a journal shadow volume, which is 
>>>> non-trivial to implement.
>>>>
>>>> Unless there is some kind of a major mitigating circumstance, it seems
>>>> that between this and the race condition that Martin is talking about 
>>>> on the other thread, GlusterFS in it's current is just too dangerous 
>>>> to use in most environments that I can think of. And unlike Gareth a 
>>>> few days ago, I'm not talking about performance issues - I'm talking 
>>>> about scope for data loss in very valid and very common use cases. 
>>>> :'(
>>>>
>>>
>>> Hmm, what about trusted.glusterfs.createtime (epoch time) as a major
>>> version number, and trusted.glusterfs.version as the minor version 
>>> number. Couple that with a glusterfs master time node (defaults to 
>>> lock node) and you should have a fairly consistent cluster, right?
>>>
>>
>>  There are several problems with this:
>>  1) The concept of the "lock node" is limiting. The locking should be
>> distributed.
>>  2) Using creation/modification time as the major number is problematic due
>> to time syncing. What happens when the master node goes offline? If the
>> nodes are in not in perfect time sync, you've still got the same problem.
>
> Correct, if machines running afrs are not time sync, it can cause problems.
> We were thinking of using parent's directories version as the file's
> createtime attribute. We increment the parent dir version first then
> create the file and apply parent's version as the file's createtime.
> Any thought on this?

Hmm... File-replacement-clobber issue only arises when a file is 
removed and another file with the same name created. It's creation version 
that's the problem. So yes, I think bumping up directory version on very 
file (a subdirectory also being a file) create/delete operation, and using 
the directory version at creation time as the major version number, with 
standard file version number as it is at the moment being the minor 
version number would work.

Good idea, and considerably simpler than what I was thinking about with 
the journalling. :-)

Gordan