Date: Sun, 22 Jun 2008 19:58:34 +0200 From: Ivan Voras <ivoras@freebsd.org> To: freebsd-geom@freebsd.org Subject: Re: Filesystem replication geom proposal Message-ID: <g3m3se$eau$1@ger.gmane.org> In-Reply-To: <0A8C1986-1DC1-4445-9111-0DEDBBCC6847@hmallett.co.uk> References: <0A8C1986-1DC1-4445-9111-0DEDBBCC6847@hmallett.co.uk>
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[-- Attachment #1 --] Hywel Mallett wrote: > FreeBSD doesn't have a defined method of replicating data between two > servers, for HA/DR purposes, similar to Veritas Volume Replicator. Linux > has DRDB (http://www.drbd.org/) which isn't quite the same, and someone > has tried using gmirror and ggate to have a mirror across teo systems, > but neither of those can cope well with a disconnected network, or a > slow network link. > I was wondering if there would be any interest in creating a new geom > provider to solve this problem. I can see that questions about this > functionality have been asked on the mailing lists before, and I've > drawn up some initial thoughts aand ideas at > http://www.hmallett.co.uk/computing-mainmenu-49/72-computing/124-open-source-replicated-filesystems.html Hmmm. If I understand your proposal, you want to create IO journals on both the master and the slave, then replicate journal data from the master to the slave, then replay the journal on the slave? This looks like a lot of work for something that looks like it could be implemented by a linked list in the kernel (to achieve aynchronous operation). The DCM looks like an alternative to the above "transactional" method of replication - it looks like instead of asynchronously replaying the transaction log, you constantly replicate the DCM to the slave (or at least the changed bits), and then the slave asks the server to send the blocks corresponding to what's marked as changed in the DCM, right? Regarding swapping the master/slave roles: I think you need a fsck step somewhere in there, or the same tricks gjournal uses (hooks into UFS) since the file system will be marked dirty if you suddenly stop using it. Also, to manually force the failover, the master needs to umount the file system, probably with "-f". Will it work? Of course, if you think the things you proposed will solve reliable replication of data across the network, go ahead :) But since ggate+gmirror has already tried to solve this, maybe you'd be interested in increasing their reliability, as an exercise before trying something from scratch? [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIXpLKldnAQVacBcgRAkDWAKCVLdvBBvhXKnCu227t/Rzk9tg6FwCgphKI Kvu6ziF/NbwjFq7juqW/p1s= =5KG3 -----END PGP SIGNATURE-----
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