From: Bruce Momjian <pgman@candle.pha.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Richard Huxton <dev@archonet.com>
Cc: pgsql-docs@postgresql.org
Subject: Re: Proposed doc-patch: Identifying the Current WAL file
Date: Sat, 15 Apr 2006 12:24:45 -0400 (EDT)
Message-ID: <200604151624.k3FGOj525540@candle.pha.pa.us> (raw)
In-Reply-To: <9932.1145030835@sss.pgh.pa.us>
Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Richard Huxton wrote:
> >> To identify the current, partially-filled WAL segment, sort first by
> >> mtime and second by file name. That is, take the latest mtime among the
> >> properly-named files, breaking ties by taking the higher filename.
>
> > I am confused by this. Why do both mtime and file name need to be
> > checked?
>
> Because recycled WAL segments are renamed to have higher file names than
> the currently-in-use segment. So you can't depend on file name first.
> However, shortly after a segment switch two WAL segments could have the
> same mtime (to within whatever the mtime granularity is, typ. 1 second).
>
> The proposed rule should be OK as long as checkpoints (and ensuing
> renames) can't occur oftener than the mtime granularity. If you're
> checkpointing more than once a second, well, you need help ...
I am trying to figure out how this could even be done in a shell script.
'ls -lt' is going to show:
-rw-r--r-- 1 root postgres 0 Apr 15 11:56 x1
-rw-r--r-- 1 root postgres 0 Apr 15 11:56 x2
-rw-r--r-- 1 root postgres 0 Apr 15 11:56 x3
but this might be with second resolution:
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:47 x1
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:47 x2
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:34 x3
or it might be:
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:47 x1
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:34 x2
-rw-r--r-- 1 root postgres 0 Apr 15 11:56:25 x3
In the first case, x2 is current, having be just switched to from x1,
while in the second case, x1 is current. In BSD, you can use ls -ltT to
see the seconds, but in Linux it is something different, and I am sure
there are some operating systems that don't allow you to see the seconds
at all. What general command-line solution can we propose for this
process? And if we can't provide one, should we supply an SQL function
to return the current WAL name?
--
Bruce Momjian http://candle.pha.pa.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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To: pgsql-docs@postgresql.org
Cc: pgman@candle.pha.pa.us, tgl@sss.pgh.pa.us, dev@archonet.com
Subject: Re: Proposed doc-patch: Identifying the Current WAL file
In-Reply-To: <200604151624.k3FGOj525540@candle.pha.pa.us>
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