Re: [WIP] Performance Improvement by reducing WAL for Update Operation

On 9 August 2012 11:30, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> On 09.08.2012 12:18, Simon Riggs wrote:
>>
>> On 9 August 2012 09:49, Amit Kapila<amit.kapila@huawei.com>  wrote:
>>
>>>    WAL reduction is done for the case even when old and new are on
>>> different
>>> data blocks as well.
>>
>>
>> That makes me feel nervous. I doubt the marginal gain is worth it.
>> Most updates don't cross blocks.
>
>
> That was my first instinctive reaction too. But if the mechanism works just
> as well for cross-page updates, seems a bit strange to not use it.
>
> One argument would be that if for some reason the old block is corrupt or
> lost, you would not be able to recover the new version of the tuple from the
> WAL alone. At the moment, it's nice that the WAL record contains all the
> information required to reconstruct the new tuple, regardless of the old
> data block contents.

Exactly. If we lose the first block in a checkpoint, we could lose all
updates to rows in that page and all other pages linked to it over a
whole checkpoint duration. Basically, page corruption will propogate
from block to block if we do this.

Given the marginal gain because of a low percentage of cross-block
updates, I'm not keen. Low percentage because HOT tries hard to keep
things on same block - even for non-HOT updates (which is the case,
even though it sounds weird).

> But then again, full-page writes cover that too. There
> will be a full-page image of the old block in the WAL anyway.

Right, but we're planning to remove that, so its not a safe assumption
to use when building new code.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services