High Availability

[Ciro Iriarte]

El abr 22, 2015 12:51 AM, "Bron Gondwana" <brong at fastmail.fm> escribió:
>
> On Wed, Apr 22, 2015, at 02:27 PM, Ciro Iriarte wrote:
>>
>> Interesting, is the use of several instances needed because cyrus cannot
scale with threads in a single instance scenario?
>
> There are two interesting reasons:
>
> 1) global locks.  There are some - mailboxes.db for example.  If you have
multiple instances on a single machine, then a lock never blocks up the
entire machine.
>
> 2) replication and load spreading - right now there's no support for
"partial replica" - a Cyrus instance replicates every mailbox to its
replica.
>
> The second one is the kicker.
>
> If we replicated everything from one machine to another machine, then
we'd have 100% user load on one machine and nothing on the other - not
efficient use of resources, because the second one needs to have the
capacity to run at 100% in a failover situation too.
>
> Our first thought was to run two instances per machine and pair them - so
there was a master on one and a replica on the other.  At least then we're
running equally in the general situation, and only in a failover situation
are we loaded 100%.  But it's still nasty - you go from 50% load to 100%
load.
>
> So we have about 10 different replicas for each machine, and every
machine is running at 50% capacity.  If we need to take one machine down,
then 10 other machines run at 55% capacity instead for that time.  The load
change is much less.
>
> (as of about a year ago, we're fully paired odd-host-number to
even-host-number, and odd and even are in different cabinets, so we can
shut down an entire cabinet by raising the load on its replicas)
>
> Bron.
>
> --
> Bron Gondwana
> brong at fastmail.fm

Hi Bron, it makes sense from that perspective although it seems to imply a
management nightmare. Do you use any management/automation (webscale if you
want) framework?.

Regards,
Ciro
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