Re: crash-safe visibility map, take three

On 30.11.2010 18:33, Tom Lane wrote:
> Robert Haas<robertmhaas(at)gmail(dot)com> writes:
>> Oh, but it's worse than that. When you XLOG a WAL record for each of
>> those pages, you're going to trigger full-page writes for all of them.
>> So now you've turned 1GB of data to write into 2+ GB of data to
>> write.
>
> No, because only the first mod of each VM page would trigger a full page
> write, at least assuming a reasonable ordering of the operations.

If you change the LSN on the heap pages, you have to write full page
images of those as well.

Let's recap what happens when a VM bit is set: You set the
PD_ALL_VISIBLE flag on the heap page (assuming it's not set already, it
usually isn't), and then set the bit in the VM while keeping the heap
page locked.

Can we get away with not setting the LSN on the heap page, even though
we set the PD_ALL_VISIBLE flag? If we don't set the LSN, the heap page
can be flushed to disk before the WAL record, but I think that's fine
because it's OK to have the flag set in the heap page even if the VM bit
is not set.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com