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Sun, 15 Jul 2007

Distributed storage suspend mode or live data migration.

I've just thought what feature I should add into this - suspend mode or live migration.
Let's say you want to change remote node - either temporary suspend all IO for given node (for example to change a local disk) or replace completely one node with another (for example switch to different remote machine), so that until data migration from one node to another or during disk replacement all block requests, which would be completed on given node, will be frozen until node is ready. Requests to the different nodes should continue without stops.
Actually I would be surprised if such functionality does not exist in existing block layer hotplug, but I do not even know how to test if it is there or not - documentation sucks, there is no feature list (at least I do not know about it), so I will reinvent the wheel (again).
There is something like queue plug and unplug, but as usual - specs suck. I will check LWN kernel line, I recall Jonathan Corbet wrote about it, but even if it does exist, it can not help in distributed storage, since it is a single device for the block layer and thus has only one queue, but stopping all IO requests because of one node is not politically correct I think. Such decision should be made by algorithm of course, since redundancy might require several nodes to be updated for single block IO, or even more - to write to some another node algorithm must read some data from suspended node, so this is the only place which knows about what IO must be frozen.

I'm thinking about should I release alpha version right now (modulo testing I need to perform for local mode) or implement some other tasty things and show distributed storage only after that... Pros and cons?

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