Re: [BUG] Checkpointer on hot standby runs without looking checkpoint_segments

On Tue, Apr 24, 2012 at 3:53 PM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 23.04.2012 02:59, Fujii Masao wrote:
>>
>> On Thu, Apr 19, 2012 at 2:20 PM, Kyotaro HORIGUCHI
>> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>  wrote:
>>>
>>> Hello, this is new version of standby checkpoint_segments patch.
>>
>>
>> Thanks for the patch!
>
>
> This still makes catching up in standby mode slower, as you get many more
> restartpoints. The reason for ignoring checkpoint_segments during recovery
> was to avoid that. Maybe it's still better than what we have currently, I'm
> not sure, but at least it needs to be discussed.

I see your point. Agreed.

Another aspect of this problem is that if we ignore checkpoint_segments during
recovery, a restartpoint would take long time, and which prevents WAL files from
being removed from pg_xlog for a while. Which might cause the disk to fill up
with WAL files. This trouble is unlikely to happen in 9.1 or before because the
archived WAL files are always restored with a temporary name. OTOH, in 9.2,
cascading replication patch changed the recovery logic so that the archived
WAL files are restored with the correct name, so the number of WAL files in
pg_xlog keeps increasing until a restartpoint removes them. The disk is more
likely to fill up, in 9.2.

To alleviate the above problem, at least we might have to change the recovery
logic so that the archived WAL files are restored with a temporary name,
if cascading replication is not enabled (i.e., !standby_mode || !hot_standby ||
max_wal_senders <= 0). Or we might have to remove the restored WAL file
after replaying it and before opening the next one, without waiting for
a restartpoint to remove the restored WAL files. Thought?

Regards,

--
Fujii Masao