Re: [PoC] Non-volatile WAL buffer

On 11/27/20 1:02 AM, Tomas Vondra wrote:
>
> Unfortunately, that patch seems to fail for me :-(
>
> The patches seem to be for PG12, so I applied them on REL_12_STABLE (all
> the parts 0001-0005) and then I did this:
>
> LIBS="-lpmem" ./configure --prefix=/home/tomas/pg-12-pmem --enable-debug
> make -s install
>
> initdb -X /opt/pmemdax/benchmarks/wal -D /opt/nvme/benchmarks/data
>
> pg_ctl -D /opt/nvme/benchmarks/data/ -l pg.log start
>
> createdb test
> pgbench -i -s 500 test
>
>
> which however fails after just about 70k rows generated (PQputline
> failed), and the pg.log says this:
>
> PANIC: could not open or mmap file
> "pg_wal/000000010000000000000006": No such file or directory
> CONTEXT: COPY pgbench_accounts, line 721000
> STATEMENT: copy pgbench_accounts from stdin
>
> Takashi-san, can you check and provide a fixed version? Ideally, I'll
> take a look too, but I'm not familiar with this patch so it may take
> more time.
>

I did try to get this working today, unsuccessfully. I did manage to
apply the 0002 part separately on REL_12_0 (there's one trivial rejected
chunk), but I still get the same failure. In fact, when built with
assertions, I can't even get initdb to pass :-(

I do get this:

TRAP: FailedAssertion("!(page->xlp_pageaddr == ptr - (ptr % 8192))",
File: "xlog.c", Line: 1813)

The values involved here are

xlp_pageaddr = 16777216
ptr = 20971520

so the page seems to be at the very beginning of the second WAL segment,
but the pointer is somewhere later. A full backtrace is attached.

I'll continue investigating this, but the xlog code is not particularly
easy to understand in general, so it may take time.

regards

--
Tomas Vondra
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