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Re: Deleted WAL files held open by backends in Linux

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Deleted WAL files held open by backends in Linux
Date: 2009-11-30 19:53:28
Message-ID: 4B13CE58020000250002CDEB@gw.wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
 
> It's not about the size of a temp table, because writes to the
> temp table itself aren't WAL-logged.  However, the system catalog
> entries for a temp table *are* WAL-logged.
 
Definitely not issuing any CREATE TEMP statements of any kind,
unless the JDBC driver is doing that under the covers.
 
>> Pretty much every read only JDBC connection seems to be holding
>> open a deleted WAL file on my Linux box, but it would take pretty
>> pessimal timing for each connection to be holding open a
>> different one -- I see that many connections share a deleted WAL
>> file.
> 
> This still seems a bit improbable to me.  There has to be
> something causing those sessions to touch WAL, and the
> dirty-buffer scenario doesn't seem reliable enough.
> 
> [ thinks... ]  How about SELECT FOR UPDATE or SELECT FOR SHARE?
> Those cause WAL writes.
 
Definitely not.
 
Probably best not to worry about it until I can play around with
some Java test code to see what it takes to cause the connection to
open the WAL.  I'll post when I've had a chance to try that.
 
-Kevin

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