[Gluster-users] active/active failover

[Alex Chekholko]

My understanding is that with GlusterFS you cannot change the volume type.
E.g. if your volume is composed of 2-way replicas, you can add more pairs;
if it's a distributed volume, you can add more bricks.  But you can't
change the type of a volume.


On Tue, Dec 12, 2017 at 2:09 PM, Stefan Solbrig <stefan.solbrig at ur.de>
wrote:

> Hi Alex,
>
> Thank you for the quick reply!
>
> Yes, I'm aware that using “plain” hardware with replication is more what
> GlusterFS is for. I cannot talk about prices where in detail, but for me,
> it evens more or less out.   Moreover, I have more SAN that I'd rather
> re-use (because of Lustre) than buy new hardware.   I'll test more to
> understand what precisely "replace-brick" changes.  I understand the mode
> of operation in case of replicated volumes.  But I was surprised (in a good
> way) that is was also working for distributed volumes, i.e., I was
> surprised that gluster does not complain of the new brick already contains
> data.  It there some technical documentation of the inner workings of
> glusterfs?
>
> This leads me to the question: If I wanted to extend my current
> installation (the one that uses SANs) with more standard hardware:   is is
> possible to mix  replicated and non-replicated bricks?  (I assume no... but
> I still dare to ask.)
>
> best wishes,
> Stefan
>
> Am 11.12.2017 um 23:07 schrieb Alex Chekholko <alex at calicolabs.com>:
>
> Hi Stefan,
>
> I think what you propose will work, though you should test it thoroughly.
>
> I think more generally, "the GlusterFS way" would be to use 2-way
> replication instead of a distributed volume; then you can lose one of your
> servers without outage.  And re-synchronize when it comes back up.
>
> Chances are if you weren't using the SAN volumes; you could have purchased
> two servers each with enough disk to make two copies of the data, all for
> less dollars...
>
> Regards,
> Alex
>
>
> On Mon, Dec 11, 2017 at 12:52 PM, Stefan Solbrig <stefan.solbrig at ur.de>
> wrote:
>
>> Dear all,
>>
>> I'm rather new to glusterfs but have some experience running lager lustre
>> and beegfs installations. These filesystems provide active/active
>> failover.  Now, I discovered that I can also do this in glusterfs, although
>> I didn't find detailed documentation about it. (I'm using glusterfs 3.10.8)
>>
>> So my question is: can I really use glusterfs to do failover in the way
>> described below, or am I misusing glusterfs? (and potentially corrupting my
>> data?)
>>
>> My setup is: I have two servers (qlogin and gluster2) that access a
>> shared SAN storage. Both servers connect to the same SAN (SAS multipath)
>> and I implement locking via lvm2 and sanlock, so I can mount the same
>> storage on either server.
>> The idea is that normally each server serves one brick, but in case one
>> server fails, the other server can serve both bricks. (I'm not interested
>> on automatic failover, I'll always do this manually.  I could also use this
>> to do maintainance on one server, with only minimal downtime.)
>>
>>
>> #normal setup:
>> [root at qlogin ~]# gluster volume info g2
>> #...
>> # Volume Name: g2
>> # Type: Distribute
>> # Brick1: qlogin:/glust/castor/brick
>> # Brick2: gluster2:/glust/pollux/brick
>>
>> #  failover: let's artificially fail one server by killing one glusterfsd:
>> [root at qlogin] systemctl status glusterd
>> [root at qlogin] kill -9 <pid/of/glusterfsd/running/brick/castor>
>>
>> # unmount brick
>> [root at qlogin] umount /glust/castor/
>>
>> # deactive LV
>> [root at qlogin] lvchange  -a n vgosb06vd05/castor
>>
>>
>> ###  now do the failover:
>>
>> # active same storage on other server:
>> [root at gluster2] lvchange  -a y vgosb06vd05/castor
>>
>> # mount on other server
>> [root at gluster2] mount /dev/mapper/vgosb06vd05-castor  /glust/castor
>>
>> # now move the "failed" brick to the other server
>> [root at gluster2] gluster volume replace-brick g2
>> qlogin:/glust/castor/brick gluster2:/glust/castor/brick commit force
>> ### The last line is the one I have doubts about
>>
>> #now I'm in failover state:
>> #Both bricks on one server:
>> [root at qlogin ~]# gluster volume info g2
>> #...
>> # Volume Name: g2
>> # Type: Distribute
>> # Brick1: gluster2:/glust/castor/brick
>> # Brick2: gluster2:/glust/pollux/brick
>>
>>
>> Is it intended to work this way?
>>
>> Thanks a lot!
>>
>> best wishes,
>> Stefan
>>
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>>
>
>
>
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