Re: 16-bit page checksums for 9.2

On Sat, Dec 24, 2011 at 2:46 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> After the various recent discussions on list, I present what I believe
>> to be a working patch implementing 16-but checksums on all buffer
>> pages.
>
> I think locking around hint-bit-setting is likely to be unworkable from
> a performance standpoint.

Anyone choosing page_checksums = on has already made a performance
reducing decision in favour of reliability. So they understand and
accept the impact. There is no locking when the parameter is off.

A safe alternative is to use LockBuffer, which has a much greater
performance impact.

I did think about optimistically checking after the write, but if we
crash at that point we will then see a block that has an invalid
checksum. It's faster but you may get a checksum failure if you crash
- but then one important aspect of this is to spot problems in case of
a crash, so that seems unacceptable.

> I also wonder whether it might not result in
> deadlocks.

If you can see how, please say. I can't see any ways for that myself.

> Also, as far as I can see this patch usurps the page version field,
> which I find unacceptably short-sighted.  Do you really think this is
> the last page layout change we'll ever make?

No, I don't. I hope and expect the next page layout change to
reintroduce such a field.

But since we're agreed now that upgrading is important, changing page
format isn't likely to be happening until we get an online upgrade
process. So future changes are much less likely. If they do happen, we
have some flag bits spare that can be used to indicate later versions.
It's not the prettiest thing in the world, but it's a small ugliness
in return for an important feature. If there was a way without that, I
would have chosen it.

pg_filedump will need to be changed more than normal, but the version
isn't used anywhere else in the server code.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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