On Wed, Oct 6, 2010 at 5:17 PM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> On 06.10.2010 11:09, Fujii Masao wrote:
>>
>> On Wed, Oct 6, 2010 at 3:31 PM, Heikki Linnakangas
>> <heikki.linnakangas@enterprisedb.com> wrote:
>>>
>>> No. Synchronous replication does not help with availability. It allows
>>> you
>>> to achieve zero data loss, ie. if the master dies, you are guaranteed
>>> that
>>> any transaction that was acknowledged as committed, is still committed.
>>
>> Hmm.. but we can increase availability without any data loss by using
>> synchronous
>> replication. Many people have already been using synchronous
>> replication softwares
>> such as DRBD for that purpose.
>
> Sure, but it's not the synchronous aspect that increases availability. It's
> the replication aspect, and we already have that. Making the replication
> synchronous allows zero data loss in case the master suddenly dies, but it
> comes at the cost of availability.
Yep. But I mean that the synchronous aspect is helpful to increase the
availability of the system which requires no data loss. In asynchronous
replication, when the master goes down, we have to salvage the missing
WAL for the standby from the failed master to avoid data loss. This would
take very long and decrease the availability of the system which doesn't
accept any data loss. Since the synchronous doesn't require such a salvage,
it can increase the availability of such a system.
If we want only no data loss, we have only to implement the wait-forever
option. But if we make consideration for the above-mentioned availability,
the return-immediately option also would be required.
In some (many, I think) cases, I think that we need to consider availability
and no data loss together, and consider the balance of them.
Regards,
--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center