Re: [vserver] Poll: High (ish) availability - how are you doing it?

From: Gordan Bobic <gordan_at_bobich.net>
Date: Sat 31 Jul 2010 - 22:59:52 BST
Message-ID: <4C549CD8.4040405@bobich.net>

Edward Capriolo wrote:
> On Fri, Jul 30, 2010 at 5:19 AM, Jeff Jansen <jeff.jansen@kkoncepts.net> wrote:
>> Eugen Leitl <eugen@leitl.org> wrote on 2010-Jul-28:
>>> Please do; I would be also quite interested as well.
>> OK, my first pass at "HA Vserver with DRBD and Heartbeat" docs are up at:
>>
>> http://www.kkoncepts.net/HA
>>
>> Comments are enabled, so you can comment on the page if you've got suggestions,
>> corrections, clarifications, etc.
>>
>> Jeff Jansen
>>
>
>>> It does not scale as well as some other solutions, but it may have other advantages that you want (maybe better locking, maybe >>better fail over support...).
>
> A little off topic, but it is important distinction between scaling
> and fail over. You really have to think hard on what your looking for.
>
> DRBD gives you disk replication Active/Passive and Active/Active 2
> nodes. Active Passive does not scale and active/active "scales" to two
> nodes which really is not scaling, in best case if you scaled a web
> server now you can handle twice the traffic, what happens when you get
> three or ten times the traffic? That solution no longer holds up.

I could be wrong, but I seem to remember that DRBD supports up to 3 nodes.

> At this point you have to look into file systems that allow multiple
> read/writes NFS or OCFS2. NFS does have locking but
> it seems to be the general case that no one was any luck with it in
> high contention situations.
> http://cyrusimap.web.cmu.edu/imapd/faq.html.

_ALL_ file systems that support concurrent access have performance
problems in high contention situations.

> OCFS2 is multi-attach file systems and it supports much stronger
> locking semantics. Great! now that we have good locks and multi-mount
> the question becomes what software is designed to work with this type
> of file system? Can we have 10 nodes running mysql and
> managing/working with the same MYD tables? It may work in theory but
> practically I do not know of anyone doing it?

You can do this with GFS/GFS2 or OCFS2. You have to set MySQL's locking
to external. But the performance suffers as in any high-contention case.
Performance of such a solution is in most cases going to be worse than a
single node with a non-concurrent-access file system.

> http://forums.mysql.com/read.php?144,205829,205829

That seems to be a weird isolated incident. There are plenty of accounts
of it working fine. Possibly a bug in the particular version of MySQL
that the poster was using.

> The only software I know of that works well and scales on OCFS2 is
> oracle, (probably because oracle corporation made both)

Indeed, OCFS was initially designed for backing Oracle databases.

> Most prominent scaling solutions cassandra, hbase, mongo, redis, hdfs.
> Do NOT work with multi-attached file systems.

That's due to performance reasons. For scaling you need performance to
scale with the number of nodes you have. No shared-everything system
will provide such performance due to the locking overheads. Most
shared-everything solutions scale inversely.

Gordan
Received on Sat Jul 31 23:00:05 2010

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