Re: sequences vs. synchronous replication

On 2022/01/12 1:07, Tomas Vondra wrote:
> I explored the idea of using page LSN a bit

Many thanks!

> The patch from 22/12 simply checks if the change should/would wait for
> sync replica, and if yes it WAL-logs the sequence increment. There's a
> couple problems with this, unfortunately:

Yes, you're right.

> So I think this approach is not really an improvement over WAL-logging
> every increment. But there's a better way, I think - we don't need to
> generate WAL, we just need to ensure we wait for it to be flushed at
> transaction end in RecordTransactionCommit().
>
> That is, instead of generating more WAL, simply update XactLastRecEnd
> and then ensure RecordTransactionCommit flushes/waits etc. Attached is a
> patch doing that - the changes in sequence.c are trivial, changes in
> RecordTransactionCommit simply ensure we flush/wait even without XID
> (this actually raises some additional questions that I'll discuss in a
> separate message in this thread).

This approach (and also my previous proposal) seems to assume that the value returned from nextval() should not be used until the transaction executing that nextval() has been committed successfully. But I'm not sure how many applications follow this assumption. Some application might use the return value of nextval() instantly before issuing commit command. Some might use the return value of nextval() executed in rollbacked transaction.

If we want to avoid duplicate sequence value even in those cases, ISTM that the transaction needs to wait for WAL flush and sync rep before nextval() returns the value. Of course, this might cause other issues like performance decrease, though.

> On btrfs, it looks like this (the numbers next to nextval are the cache
> size, with 1 being the default):
>
> client test master log-all page-lsn log-all page-lsn
> -------------------------------------------------------------------
> 1 insert 829 807 802 97% 97%
> nextval/1 16491 814 16465 5% 100%
> nextval/32 24487 16462 24632 67% 101%
> nextval/64 24516 24918 24671 102% 101%
> nextval/128 32337 33178 32863 103% 102%
>
> client test master log-all page-lsn log-all page-lsn
> -------------------------------------------------------------------
> 4 insert 1577 1590 1546 101% 98%
> nextval/1 45607 1579 21220 3% 47%
> nextval/32 68453 49141 51170 72% 75%
> nextval/64 66928 65534 66408 98% 99%
> nextval/128 83502 81835 82576 98% 99%
>
> The results seem clearly better, I think.

Thanks for benchmarking this! I agree that page-lsn is obviously better than log-all.

> For "insert" there's no drop at all (same as before), because as soon as
> a transaction generates any WAL, it has to flush/wait anyway.
>
> And for "nextval" there's a drop, but only with 4 clients, and it's much
> smaller (53% instead of 97%). And increasing the cache size eliminates
> even that.

Yes, but 53% drop would be critial for some applications that don't want to increase the cache size for some reasons. So IMO it's better to provide the option to enable/disable that page-lsn approach.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION