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Re: xlog location arithmetic

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, Magnus Hagander <magnus(at)hagander(dot)net>, Euler Taveira de Oliveira <euler(at)timbira(dot)com>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: xlog location arithmetic
Date: 2012-03-13 03:36:45
Message-ID: CAHGQGwHWn11_eQsR6Hw4ko8t2eT2=vPkb-L69jvrLMhquTtMWw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Mar 13, 2012 at 12:03 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
>> On Sat, Mar 10, 2012 at 5:04 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> The main actual simplification would be in getting rid of the "hole"
>>> at the end of each 4GB worth of WAL, cf this bit in xlog_internal.h:
>>> If we can't get rid of that and have a continuous 64-bit WAL address
>>> space then it's unlikely we can actually simplify any logic.
>>> ...
>>> Now, doing that doesn't break the naming convention exactly; what it
>>> changes is that there will be WAL files numbered xxxFFFF (for some
>>> number of trailing-1-bits I'm too lazy to work out at the moment) where
>>> before there were not.  So the question really is how much external code
>>> there is that is aware of that specific noncontiguous numbering behavior
>>> and would be broken if things stopped being that way.
>
>> A page header contains WAL location, so getting rid of "hole" seems to
>> break pg_upgrade. No?
>
> No, why would it do that?  The meaning and ordering of WAL addresses is
> the same as before.  The only difference is that after the upgrade, the
> system will stop skipping over 16MB of potentially usable WAL addresses
> at the end of each subsequently-used 4GB of space.  The holes before
> the switchover point are still holes, but that doesn't matter.

Oh, I see. You're right.

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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