Re: Proposal for 9.1: WAL streaming from WAL buffers

On Tue, Jun 15, 2010 at 8:09 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
>
>> I have yet to convince myself of how likely this is to occur.  I tried
>> to reproduce this issue by crashing the database, but I think in 9.0
>> you need an actual operating system crash to cause this problem, and I
>> haven't yet set up an environment in which I can repeatedly crash the
>> OS.  I believe, though, that in 9.1, we're going to want to stream
>> from WAL buffers as proposed in the patch that started out this
>> thread, and then I think this issue can be triggered with just a
>> database crash.
>
> Yes, but it still requires:
>
> a) the master must crash with at least one transaction transmitted to
> the slave an not yet fsync'd

Bzzzzt. Stop right there. It only requires the master to crash with
at least one *WAL record* written but not transmitted, not one
transaction. And most WAL record types are not fsync'd immediately.
So in theory I think that, for example, an OS crash in the middle of a
big bulk insert operation should be sufficient to trigger this.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company