| From: | Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> |
|---|---|
| To: | "movead(dot)li(at)highgo(dot)ca" <movead(dot)li(at)highgo(dot)ca>, pgsql-hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: POC and rebased patch for CSN based snapshots |
| Date: | 2020-06-19 05:54:26 |
| Message-ID: | 5889e9df-1122-42ae-509b-ecf21370dc86@oss.nttdata.com |
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| Lists: | pgsql-hackers |
On 2020/06/19 13:36, movead(dot)li(at)highgo(dot)ca wrote:
>
> >You mean that the last generated CSN needs to be WAL-logged because any smaller
>>CSN than the last one should not be reused after crash recovery. Right?
> Yes that's it.
>
>>If right, that WAL-logging seems not necessary because CSN mechanism assumes
>>CSN is increased monotonically. IOW, even without that WAL-logging, CSN afer
>>crash recovery must be larger than that before. No?
>
> CSN collected based on time of system in this patch, but time is not reliable all the
> time. And it designed for Global CSN(for sharding) where it may rely on CSN from
> other node , which generated from other machine.
>
> So monotonically is not reliable and it need to keep it's largest CSN in wal in case
> of crash.
Thanks for the explanaion! Understood.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
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