Re: 16-bit page checksums for 9.2

On Tue, Feb 07, 2012 at 08:58:59PM +0000, Simon Riggs wrote:
> On Thu, Jan 26, 2012 at 8:20 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > On Wed, Jan 11, 2012 at 10:12:31PM +0000, Simon Riggs wrote:

> > This patch uses FPIs to guard against torn hint writes, even when the
> > checksums are disabled. ?One could not simply condition them on the
> > page_checksums setting, though. ?Suppose page_checksums=off and we're hinting
> > a page previously written with page_checksums=on. ?If that write tears,
> > leaving the checksum intact, that block will now fail verification. ?A couple
> > of ideas come to mind. ?(a) Read the on-disk page and emit an FPI only if the
> > old page had a checksum. ?(b) Add a checksumEnableLSN field to pg_control.
> > Whenever the cluster starts with checksums disabled, set the field to
> > InvalidXLogRecPtr. ?Whenever the cluster starts with checksums enabled and the
> > field is InvalidXLogRecPtr, set the field to the next LSN. ?When a checksum
> > failure occurs in a page with LSN older than the stored one, emit either a
> > softer warning or no message at all.
>
> We can only change page_checksums at restart (now) so the above seems moot.
>
> If we crash with FPWs enabled we repair any torn pages.

There's no live bug, but that comes at a high cost: the patch has us emit
full-page images for hint bit writes regardless of the page_checksums setting.

> > PageSetLSN() is not atomic, so the shared buffer content lock we'll be holding
> > is insufficient.
>
> Am serialising this by only writing PageLSN while holding buf hdr lock.

That means also taking the buffer header spinlock in every PageGetLSN() caller
holding only a shared buffer content lock. Do you think that will pay off,
versus the settled pattern of trading here your shared buffer content lock for
an exclusive one?

> > I can see value in an option to exclude local buffers, since corruption there
> > may be less exciting. ?It doesn't seem important for an initial patch, though.
>
> I'm continuing to exclude local buffers. Let me know if that should change.

Seems reasonable.

Thanks,
nm