On 31/08/2024 19:09, Kyotaro Horiguchi wrote:
- UNDO log(0002): This handles file deletion during transaction aborts,
which was previously managed, in part, by the commit XLOG record at
the end of a transaction.
- Prevent orphan files after a crash (0005): This is another use-case
of the UNDO log system.
Nice, I'm very excited if we can fix that long-standing issue! I'll try
to review this properly later, but at a quick 5 minute glance, one thing
caught my eye:
This requires fsync()ing the per-xid undo log file every time a relation
is created. I fear that can be a pretty big performance hit for
workloads that repeatedly create and drop small tables. Especially if
they're otherwise running with synchronous_commit=off. Instead of
flushing the undo log file after every write, I'd suggest WAL-logging
the undo log like regular relations and SLRUs. So before writing the
entry to the undo log, WAL-log it. And with a little more effort, you
could postpone creating the files altogether until a checkpoint happens,
similar to how twophase state files are checkpointed nowadays.
I wonder if the twophase state files and undo log files should be merged
into one file. They're similar in many ways: there's one file per
transaction, named using the XID. I haven't thought this fully through,
just a thought..
+static void
+undolog_set_filename(char *buf, TransactionId xid)
+{
+ snprintf(buf, MAXPGPATH, "%s/%08x", SIMPLE_UNDOLOG_DIR, xid);
+}
I'd suggest using FullTransactionId. Doesn't matter much, but seems like
a good future-proofing.
--
Heikki Linnakangas
Neon (https://neon.tech)
