From: | Daniel Farina <daniel(at)heroku(dot)com> |
---|---|
To: | Greg Smith <greg(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: disposition of remaining patches |
Date: | 2011-02-25 08:14:18 |
Message-ID: | AANLkTiny3piBqS_FAf9gN-Ws5Ok06Y7xwoMD4iRQqL=_@mail.gmail.com |
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Lists: | pgsql-hackers |
On Wed, Feb 23, 2011 at 11:49 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> Robert Haas wrote:
>>>
>>> 2. Synchronous replication. Splitting up this patch has allowed some
> On top of 4 listed reviewers I know Dan Farina is poking at the last update,
> so we may see one more larger report on top of what's already shown up. And
> Jaime keeps kicking the tires too. What Simon was hoping is that a week of
> others looking at this would produce enough feedback that it might be
> possible to sweep the remaining issues up soon after he's back. It looks to
> me like that's about when everything else that's still open will probably
> settle too.
Besides some of the fixable issues, I am going to have to echo
Robert's sentiments about a few kinks that go beyond mechanism in the
syncrep patch: in particular, it will *almost* solve the use case I
was hoping to solve: a way to cleanly perform planned switchovers
between machines with minimal downtime and no lost data. But there are
a couple of holes I have thought of so far:
1. The 2-safe methodology supported is not really compatible with
performing planned-HA-switchover of a cluster with its own syncrep
guarantees on top of that. For example:
Server A syncreps to Server B
Now I want to provision server A-prime, which will eventually take the
place of A.
Server A syncreps to Server B
Server A syncreps to Server A-prime
Right now, as it stands, the syncrep patch will be happy as soon as
the data has been fsynced to either B or A-prime; I don't think we can
guarantee at any point that A-prime can become the leader, and feed B.
2. The unprivileged user can disable syncrep, in any situation. This
flexibility is *great*, but you don't really want people to do it when
one is performing the switchover. Rather, in a magical world we'd hope
that disabling syncrep would just result in not having to
synchronously commit to B (but, in this case, still synchronously
commit to A-prime)
In other words, to my mind, you can use syncrep as-is to provide
2-safe durability xor a scheduled switchover: as soon as someone wants
both, I think they'll have some trouble. I do want both, though.
--
fdr
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