Re: crash-safe visibility map, take three

On Tue, Nov 30, 2010 at 11:22 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Nov 30, 2010 at 11:16 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> How much is "quite a lot"?  Do we have any real reason to think that
>> this solution is unacceptable performance-wise?
>
> Well, let's imagine a 1GB insert-only table.  It has 128K pages.  If
> you XLOG setting the bit on each page, you'll need to write 128K WAL
> records, each containing a 12-byte relfilenode and a 4-byte block
> offset, for a total of 16 bytes of WAL per page, thus 2MB of WAL.
>
> But you did just dirty a gigabyte of data.

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.

--
Robert Haas
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