On 11/12/2012 04:44 PM, Markus Wanner wrote:
> Jeff,
>
> On 11/10/2012 12:08 AM, Jeff Davis wrote:
>> The bit indicating that a checksum is present may be lost due to
>> corruption.
> Hm.. I see.
>
> Sorry if that has been discussed before, but can't we do without that
> bit at all? It adds a checksum switch to each page, where we just agreed
> we don't event want a per-database switch.
>
> Can we simply write a progress indicator to pg_control or someplace
> saying that all pages up to X of relation Y are supposed to have valid
> checksums?
That'll make it hard for VACUUM, hint-bit setting, etc to
opportunistically checksum pages whenever they're doing a page write anyway.
Is it absurd to suggest using another bitmap, like the FSM or visibility
map, to store information on page checksumming while checksumming is
enabled but incomplete? As a much smaller file the bitmap could its self
be very quickly generated in one pass when checksumming is enabled, with
its starting state showing no pages having checksums.
It perhaps its self have page checksums since presumably the persistent
maps like the FSM and visibility map will support them? Some way to
ensure the checksum map is valid would be needed.
--
Craig Ringer
