[DRBD-user] 3-node active/active/active config?

Jiann-Ming Su sujiannming at gmail.com
Wed Nov 18 21:07:21 CET 2009


On Wed, Nov 18, 2009 at 4:19 AM, Lars Ellenberg
<lars.ellenberg at linbit.com> wrote:
> On Tue, Nov 17, 2009 at 11:30:04PM -0500, Jiann-Ming Su wrote:
>> On Tue, Nov 17, 2009 at 12:50 PM, Lars Ellenberg
>> <lars.ellenberg at linbit.com> wrote:
>> >
>> > No. You did not understand.
>> >
>> > It is not a question of performance.
>> > Or whether a write reached all *nodes*.
>> >
>> > In your setup, it is technically *impossible*
>> > for a write to reach all lower level *disks*.
>> >
>>
>> Can you give a brief explanation of why that is the case?
>
> I thought I already did?
>

Gianluca's explanation cleared it up for me.

>> Is the problem the dual path?  What if a single path was used in some
>> stacked configuration where a drbd is used as a backing device for
>> another drbd share?
>
> "Stacked DRBD", three (or four) node setups, are perfecly fine and
> supported.  It is NOT possible to have more than two nodes _active_
> though.  See the User's Guide or contact LINBIT for details.
>

Ah, okay.  Thanks for clarifying that.

>> > A sure way to data corruption.
>> >
>> > Got me this time?
>> >
>> >  ;)
>> >
>> > So by all means: use one iSCSI on DRBD cluster,
>> > and have any number of ocfs2 clients via iSCSI.
>> > Or double check if NFS can do the trick for you.
>> >
>>
>> We have three geographically independent locations
>
> Who is "We", and where are those locations?
> How far appart? Network latency? Bandwidth?

Gig attached, less than 5ms ping times.

> Data Volume?

Less than 1GB.

> Approximate number of Files? Directories? Files per Directory?

Roughly 5000-10000 files and directories combined.

> Average and peak _file_ creation/deletion/modification rate?

Over a day, as low as 500 files/hr up to 10000 files/hr modification rate.

> Average _data_ change rate?
> Peak data change rate?
>
>> that have to share data, but still remain independent.
>
> And you think you want to drive one of the currently available
> cluster filesystem in some "geographically dispersed" mode.
>
> Yeahright.
>
> Cluster file systems are latency critical.
>

For this application, the filesystem performance, specifically writes,
is not that critical.  What's important is data replication.  We're
much more interested in the ability to write/modify files from any of
the three nodes.

> Even if you'd get the all-active-fully-meshed-replication to work (we at
> LINBIT are working on adding that functionality in some later version of
> DRBD), latency will kill your performance.
>

Performance is in the eye of the beholder... ;-)

> And whenever you have a network hickup, you'd have no availability,
> because you'd need to reboot at least one node.
>

Yeah, that's one of the nice things about the two node config.  It's
relatively resilient to network issues.

> I'm sure you have an interessting setup.
> But to save you a lot of time experimenting with things that simply
> won't work outside the lab, or possibly not even there, I think you
> really could use some consultancy  ;)
>

Yeah, that's why I asked here first. :-)  Thanks for all your insights.


-- 
Jiann-Ming Su
"I have to decide between two equally frightening options.
 If I wanted to do that, I'd vote." --Duckman
"The system's broke, Hank.  The election baby has peed in
the bath water.  You got to throw 'em both out."  --Dale Gribble
"Those who vote decide nothing.
Those who count the votes decide everything.”  --Joseph Stalin


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