| From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: crash-safe visibility map, take three |
| Date: | 2010-11-30 16:27:30 |
| Message-ID: | 4CF525F2.3030806@enterprisedb.com |
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| Lists: | pgsql-hackers |
On 30.11.2010 18:22, Robert Haas 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.
Plus WAL headers, I think it's something like 32 or 40 bytes of WAL per
page.
> But you did just dirty a gigabyte of data.
Good point.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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