| From: | Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com> |
|---|---|
| To: | Stephen Frost <sfrost(at)snowman(dot)net> |
| Cc: | Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Ryan Murphy <ryanfmurphy(at)gmail(dot)com>, Beena Emerson <memissemerson(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Re: Clarifying "server starting" messaging in pg_ctl start without --wait |
| Date: | 2017-01-19 14:38:14 |
| Message-ID: | add8e55c-21fc-2af6-06e2-a64441c602bc@2ndquadrant.com |
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| Lists: | pgsql-hackers |
On 1/18/17 3:12 PM, Stephen Frost wrote:
> I don't understand what I'm missing when it comes to checkpoint_timeout
> and the time required to recover from a crash. You aren't the first
> person to question that association, but it seems pretty clear to me.
>
> When doing recovery, we have to replay everything since the last
> checkpoint. If we are checkpointing at least every 5 minutes then we
> can't have any more than 5 minutes worth of WAL to replay, right?
But writing WAL and replaying WAL are two entirely different operations.
Writing a WAL record involves writing a few bytes sequentially.
Replaying a WAL record might involve hopping all over the system and
applying the changes that the WAL record describes.
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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