Re: Patch: Write Amplification Reduction Method (WARM)

On Fri, Mar 24, 2017 at 12:25 AM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
>
>
> On Thu, Mar 23, 2017 at 7:53 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
>>
>> >
>>
>> I am not sure on what basis user can set such parameters, it will be
>> quite difficult to tune those parameters. I think the point is
>> whatever threshold we keep, once that is crossed, it will perform two
>> scans for all the indexes.
>
>
> Well, that applies to even vacuum parameters, no?
>

I don't know how much we can directly compare the usability of the new
parameters you are proposing here to existing parameters.

> The general sense I've got
> here is that we're ok to push some work in background if it helps the
> real-time queries, and I kinda agree with that.
>

I don't think we can define this work as "some" work, it can be a lot
of work depending on the number of indexes. Also, I think for some
cases it will generate maintenance work without generating benefit.
For example, when there is one index on a table and there are updates
for that index column.

> Having said that, I see many ways we can improve on this later. Like we can
> track somewhere else information about tuples which may have received WARM
> updates (I think it will need to be a per-index bitmap or so) and use that
> to do WARM chain conversion in a single index pass.
>

Sure, if we have some way to do it in a single pass or does most of
the time in foreground process (like we have some dead marking idea
for indexes), then it would have been better.

> But this is clearly not
> PG 10 material.
>

I don't see much discussion about this aspect of the patch, so not
sure if it is acceptable to increase the cost of vacuum. Now, I don't
know if your idea of GUC's make it such that the additional cost will
occur seldom and this additional pass has a minimal impact which will
make it acceptable.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com