Re: Enabling Checksums

On Thu, Jan 10, 2013 at 1:06 AM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> On Tue, 2012-12-04 at 01:03 -0800, Jeff Davis wrote:
>> For now, I rebased the patches against master, and did some very minor
>> cleanup.
>
> I think there is a problem here when setting PD_ALL_VISIBLE. I thought I
> had analyzed that before, but upon review, it doesn't look right.
> Setting PD_ALL_VISIBLE needs to be associated with a WAL action somehow,
> and a bumping of the LSN, otherwise there is a torn page hazard.
>
> The solution doesn't seem particularly difficult, but there are a few
> strange aspects and I'm not sure exactly which path I should take.
>
> First of all, the relationship between MarkBufferDirty and
> SetBufferCommitInfoNeedsSave is a little confusing. The comment over
> MarkBufferDirty is confusing because it says that the caller must have
> an exclusive lock, or else bad data could be written. But that doesn't
> have to do with marking the buffer dirty, that has to do with the data
> page change you make while you are marking it dirty -- if it's a single
> bit change, then there is no risk that I can see.
>
> In the current code, the only real code difference between the two is
> that SetBufferCommitInfoNeedsSave might fail to mark the buffer dirty if
> there is a race. So, in the current code, we could actually combine the
> two by passing a "force" flag (if true, behaves like MarkBufferDirty, if
> false, behaves like SetBufferCommitInfoNeedsSave).
>
> The checksums patch also introduces another behavior into
> SetBufferCommitInfoNeedsSave, which is to write an XLOG_HINT WAL record
> if checksums are enabled (to avoid torn page hazards). That's only
> necessary for changes where the caller does not write WAL itself and
> doesn't bump the LSN of the data page. (There's a reason the caller
> can't easily write the XLOG_HINT WAL itself.) So, we could introduce
> another flag "needsWAL" that would control whether we write the
> XLOG_HINT WAL or not (only applies with checksums on, of course).

I thought Simon had the idea, at some stage, of writing a WAL record
to cover hint-bit changes only at the time we *write* the buffer and
only if no FPI had already been emitted that checkpoint cycle. I'm
not sure whether that approach was sound, but if so it seems more
efficient than this approach.

--
Robert Haas
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