On Thu, 15 Jun 2000, Andrew Gillham wrote:

> Under the Qmail FAQ there is question of reliability.  It says not
> to use softupdates with qmail.  Why is this a problem?  I thought
> that that all filesystem operations were guaranteed to be safe by
> the "ordered" nature of soft updates?

What soft updates do is reduce the probablility of fs inconsistancy due to
the fs going down while changes have been defered. So you can get the
performance boost of defering metadata writes without the inconsistancy
vulnerability.

Softupdates doesn't ensure that things which are waiting to be written are
written when the machine goes down, just that the fs is much less likely
to become inconsistant.

QMail is concerned that it can't tell if a message is waiting to be
written to disk or has really been written to disk. It can't tell the
difference, and doesn't want to get into the situation where it thinks an
EMail has been written to disk and it hasn't. Thus if the machine dies at
that point, that EMail is gone. As that was the only copy, there's no way
to get it back.

Take care,

Bill