Re: crash-safe visibility map, take three
| От | Robert Haas |
|---|---|
| Тема | Re: crash-safe visibility map, take three |
| Дата | |
| Msg-id | AANLkTinVXHXSi0Bgtz5_WHPnOqGcZj_DT6cB2c2_HS3Z@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: crash-safe visibility map, take three (Robert Haas <robertmhaas@gmail.com>) |
| Ответы |
Re: crash-safe visibility map, take three
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| Список | pgsql-hackers |
On Tue, Nov 30, 2010 at 11:22 AM, Robert Haas <robertmhaas@gmail.com> wrote: > On Tue, Nov 30, 2010 at 11:16 AM, Tom Lane <tgl@sss.pgh.pa.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+ GBof data to write. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
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