Re: Reviewing freeze map code

On Fri, Jun 24, 2016 at 11:04 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Fri, Jun 24, 2016 at 4:33 AM, Andres Freund <andres@anarazel.de> wrote:
>> On 2016-06-23 18:59:57 -0400, Alvaro Herrera wrote:
>>> Andres Freund wrote:
>>>
>>> > I'm looking into three approaches right now:
>>> >
>>> > 3) Use WAL logging for the already_marked = true case.
>>>
>>>
>>> > 3) This approach so far seems the best. It's possible to reuse the
>>> > xl_heap_lock record (in an afaics backwards compatible manner), and in
>>> > most cases the overhead isn't that large.  It's of course annoying to
>>> > emit more WAL, but it's not that big an overhead compared to extending a
>>> > file, or to toasting.  It's also by far the simplest fix.
>>>
>
> +1 for proceeding with Approach-3.
>
>>> I suppose it's fine if we crash midway from emitting this wal record and
>>> the actual heap_update one, since the xmax will appear to come from an
>>> aborted xid, right?
>>
>> Yea, that should be fine.
>>
>>
>>> I agree that the overhead is probably negligible, considering that this
>>> only happens when toast is invoked.  It's probably not as great when the
>>> new tuple goes to another page, though.
>>
>> I think it has to happen in both cases unfortunately. We could try to
>> add some optimizations (e.g. only release lock & WAL log if the target
>> page, via fsm, is before the current one), but I don't really want to go
>> there in the back branches.
>>
>
> You are right, I think we can try such an optimization in Head and
> that too if we see a performance hit with adding this new WAL in
> heap_update.
>
>

+1 for #3 approach, and attached draft patch for that.
I think attached patch would fix this problem but please let me know
if this patch is not what you're thinking.

Regards,

--
Masahiko Sawada