Re: SLRUs in the main buffer pool - Page Header definitions

> Unfortunately a quite large patch, with this little explanation, is
> hard to review. I could read through the entire thread to try to 
> figure out what this is doing, but I really shouldn't have to.
 
> You're changing quite fundamental APIs across the tree. Why is that 
> required for the topic at hand? Why is it worth doing that, given 
> it'll break loads of extensions?
 
Hi Andres, 

Thanks for your response.

To summarize, our underlying effort is to move the SLRUs to the buffer 
cache. We were working with Thomas Munro off a patch he introduced here
[1]. Munro’s patch moves SLRUs to the buffer cache by introducing the
pseudo db id 9 to denote SLRU pages, but maintains the current “raw”
data format of SLRU pages. The addition of page headers in our patch
resolves this issue [2] Munro mentions in this email [3]. 

Heikki Linnakangas then introduced  patch on top of Munro’s patch that 
modularizes the storage manager, allowing SLRUs to use it [4]. Instead
of using db id 9, SLRUs use spcOid 9, and each SLRU is its own relation.
Here, Heikki simplifies the storage manager by having each struct be 
responsible for just one fork of a relation; thus increasing
extensibility of the smgr API, including for SLRUs. [5] We integrated
our changes introducing page headers for SLRU pages, and upgrade logic
to Heikki’s latest patch. 
 
> > Rebased patch as per latest community changes since last email.

> This version doesn't actually build.
> https://cirrus-ci.com/task/4512310190931968
> [19:43:20.131] FAILED: 
> src/test/modules/test_slru/test_slru.so.p/test_slru.c.o


As for the build failures, I was using make install to build, and make
check for regression tests. The build failures in this link  are from
unit tests dependent on custom SLRUs, which would no longer apply. I’ve
attached another patch that removes these tests.

[1] https://www.postgresql.org/message
-id/CA%2BhUKGKCkbtOutcz5M8Z%3D0pgAkwdiR57Lxk7803rGsgiBNST6w%40mail.gmail
.com

[2] “I needed to disable checksums and in-page LSNs, since SLRU pages
hold raw data with no header. We'd probably eventually want regular
(standard? formatted?) pages (the real work here may be implementing
FPI for SLRUs so that checksums don't break your database on torn
writes). ” 

[3] https://www.postgresql.org/message-id/CA%2BhUKGKAYze99B-jk9NoMp-
2BDqAgiRC4oJv%2BbFxghNgdieq8Q%40mail.gmail.com

[4] https://www.postgresql.org/message-id/flat/128709bc-992c-b57a-7174-
098433b7faa4%40iki.fi#a78d6250327795e95b02e9305e2d153e

[5] “I think things would be more clear if we 
unbundled the forks at the SMGR level, so that we would have a separate 
SMgrRelation struct for each fork. And let's rename it to SMgrFile to 
make the role more clear. I think that would reduce the confusion when 
we start using it for SLRUs; an SLRU is not a relation, after all. md.c 
would still segment each logical file into 1 GB segments, but it would 
not need to deal with forks.”