[TriLUG] Replicating a filesystem across datacenters

[tj]

As I understand based on using DRBD +heartbeat from my past works.

DRBD is a neat functionality for only in LAN(limited WAN scenario such as:
building 1 to building XYZ in the same campus or not far distance), since
we can set rule which machine will take control(automatically or manually)
when heartbeat is not detected or loss.

as Igor mentioned, latency is a killing bee for DRBD,

DRBD can be setup as UDP that assumes whom ever secondary DRDB always
accepting data from primary DRBD with one culprit: acting master always
assumes secondary always gets full update since DRBD on UDP does not give
100% sync updated data, which is not true when latency and lost data occurs.



On Tue, Nov 4, 2014 at 7:54 PM, Igor Partola <igor at igorpartola.com> wrote:

> I have not worked with file systems or distributed block devices, but I
> have worked with WAN distributed databases. Even that narrow use case has
> enough depth to it that building a solution took quite a bit of research
> and custom code.
>
> The basic problem is speed of light. Going from DC to Dallas (that was my
> use case) meant a 30 ms RTT. This means that effectively your disks are
> suddenly very slow. Add the fact that partitions do happen, and that the
> network performance is not stable, and I think you can rule out pretty much
> any block device setup. The code written to run over block devices assumes
> some basic latency and predictable failure modes. This is similar to how
> stuff randomly hangs when you have wonky networking and NFS.
>
> Also, distributed block devices cannot be shared: the original use case
> here is that there are writers in both data centers. If anything, some
> magical file system might do this.
>
> More realistically, I would looks at what your applications are doing, if
> you can modify them and see if you can add multi-master support there. This
> is a horribly in-UNIX-y solution, but it is the current best approach for a
> reason: magic bullets don't exist so you have to create your own.
>
> The only real alternative is hot standby. This is more reliable for the
> general case but much less exciting.
>
> Igor
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