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Re: [Gluster-devel] trusted.glusterfs.version xattr

From: Kevan Benson
Subject: Re: [Gluster-devel] trusted.glusterfs.version xattr
Date: Tue, 06 May 2008 15:38:41 -0700
User-agent: Thunderbird (X11/20071220)

Gordan Bobic 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?

I seem to remember toying with this idea a few months ago on this list, for a different problem. I can't recall whether it was shot full of holes at that time (and I guess I'm too lazy to search the archives before posting ;)


-Kevan Benson
-A-1 Networks

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