| From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Samrat Revagade <revagade(dot)samrat(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Patch for fail-back without fresh backup |
| Date: | 2013-06-14 13:17:25 |
| Message-ID: | 51BB17E5.3060509@vmware.com |
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| Lists: | pgsql-hackers |
On 14.06.2013 16:08, Tom Lane wrote:
> Heikki Linnakangas<hlinnakangas(at)vmware(dot)com> writes:
>> Well, time will tell I guess. The biggest overhead with the checksums is
>> exactly the WAL-logging of hint bits.
>
> Refresh my memory as to why we need to WAL-log hints for checksumming?
Torn pages:
1. Backend sets a hint bit, dirtying the buffer.
2. Checksum is calculated, and buffer is written out to disk.
3. <crash>
If the page is torn, the checksum won't match. Without checksums, a torn
page is not a problem with hint bits, as a single bit can't be torn and
the page is otherwise intact. But with checksums, it causes a checksum
failure.
- Heikki
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