Re: Tuning for PostGreSQL Database

From: Terry Lambert (tlambert2_at_mindspring.com)
Date: 07/23/03

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    Date: Tue, 22 Jul 2003 23:14:05 -0700
To: Simon J Mudd <sjmudd@pobox.com>

    Simon J Mudd wrote:
    > Someone else mentioned Qmail. I use Postfix and I know the author
    > relies on fsync semantics before confirming the acceptance of email.
    > I'm sure Qmail (and sendmail, ...) does the same. They should do -
    > the RFC requires this (that mail can't be lost once it is accepted for
    > delivery).

    It's OK t fsync() the data file before sending back the "250 OK";
    RFC 821 (and now 2821) require this behavior: the data much be
    committed to stable storage before the "250 OK" response is sent
    (unless you are HotMail, of course ;^)).

    The problem with qmail is that it assumes that it needs to sync
    the directory because it thinks the underlying FS implemention
    violates the POSIX standard with regard to "SHALL be updated"
    instead of "SHALL be marked for update" semantics on directory
    metadata. So qmail does an extra fsync() on something that
    doesn't need to be fsync()'ed.

    This makes qmail unnecessarily slow, in order to make it safe
    for people to run nominally POSIX code on non-POSIX compliant
    systems.

    For POSIX systems, qmail should turn this behaviour off.

    Unfortunately, there are a number of systems which define POSIX
    manifest constants, thus lying about whether or not they are
    really POSIX compliant systems.

    -- Terry
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