Re: [Xen-API] cross-pool migrate, with any kind of storage (shared or local)

[George Shuklin <george.shuklin@xxxxxxxxx>]

Wow! Great news!

And idea: why not use DRBD? It suits perfectly for replication changes
between two block devices, it will do all work, includes an dirty map
tracking, synchronous writing and replicating with controllable speed.

It also have different protocols for new writes replication: sync and
async - perfect for any kind of replication.

DRBD allows to keep  metadata  separately from media (disk is untouched
by drbd during operating).

The main disadvantage of DRBD is just and only TWO nodes - but this is
perfectly suites for task 'replicate from ONE node to SECOND').

And DRBD gurantee consistency between nodes, and it even supports
primary-primary mode, which allow as to make migration lag (when VM not
operates) minimal.

And it supports online reconfiguration of peer!

I see no more perfect solution for this: it's already in product, in
vanilla kernel and it have everything we need.



Ð ÐÑÐ, 13/07/2011 Ð 15:21 +0100, Dave Scott ÐÐÑÐÑ:
> Hi,
> 
> I've created a page on the wiki describing a new migration protocol for xapi. 
> The plan is to make migrate work both within a pool and across pools, and to 
> work with and without storage i.e. transparently migrate storage if necessary.
> 
> The page is here:
> 
> http://wiki.xensource.com/xenwiki/CrossPoolMigration
> 
> The rough idea is to:
> 1. use an iSCSI target to export disks from the receiver to the transmitter
> 2. use tapdisk's log dirty mode to build a continuous disk copy program
> -- perhaps we should go the full way and use the tapdisk block mirror code to 
> establish a full storage mirror?
> 3. use the VM metadata export/import to move the VM metadata between pools
> 
> I'd also like to
> * make the migration code unit-testable (so I can test the failure paths 
> easily)
> * make the code more robust to host failures by host heartbeating
> * make migrate properly cancellable
> 
> I've started making a prototype-- so far I've written a simple python wrapper 
> around the iscsi target daemon:
> 
> https://github.com/djs55/iscsi-target-manager
> 
> _______________________________________________
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