Re: BUG #5741: syslog line length

On Fri, Jun 10, 2011 at 04:21:59PM +0200, Marc Cousin wrote:
> On 02/11/2010 17:17, heasley wrote:
> > The following bug has been logged online:
> >
> > Bug reference: 5741
> > Logged by: heasley
> > Email address: heas(at)shrubbery(dot)net
> > PostgreSQL version: 8.4
> > Operating system: solaris
> > Description: syslog line length
> > Details:
> >
> > * Max string length to send to syslog(). Note that this doesn't count the
> > * sequence-number prefix we add, and of course it doesn't count the prefix
> > * added by syslog itself. On many implementations it seems that the
> > hard
> > * limit is approximately 2K bytes including both those prefixes.
> > */
> > #ifndef PG_SYSLOG_LIMIT
> > #define PG_SYSLOG_LIMIT 1024
> > #endif
> >
> > solaris' syslogd limits the line length to 1024, with a
> > FQDN and it's silly "msg ID" quite a bit is dropped by
> > syslogd.
> >
> I've been having the exact same problem with CentOS 5.5 these days (and
> a customer's Red Hat 5.4).
>
> This same problem occurs with sysklogd, which has a
> #define MAXLINE 1024 /* maximum line length */

A PG_SYSLOG_LIMIT value that loses data to truncation on nearly every default
GNU/Linux installation makes for a poor default. It seems we raised it from 128
to 1024 for version 8.4, on performance grounds:
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=6b7eebc05e3bfd86757f5aa1f05bca463e92d7db

How about 896? I reach that figure based on 1024 as a reasonable limit for the
total syslog line, including prefix, and these estimates for the prefix parts:

25 - fixed-length content
16 - syslog_ident [default is "postgres"]
10 - pid
32 - hostname [certainly not a limit, but frequently adequate]
10 - sequence number [ditto]
3 - chunk number [ditto]
32 - facility/priority/message ID added by some syslog implementations
= 128

Perhaps, though, the default should be outright conservative, like 512. We
could also have a compile-time fixed overhead and factor in actual lengths of
parts we know at runtime, but that smells like overkill. Opinions?

--
Noah Misch http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services