On 14.06.2013 16:08, Tom Lane wrote:
> Heikki Linnakangas<hlinnakangas@vmware.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