[Soekris] SMTP on Soekris?
kudzu at tenebras.com
Tue Feb 4 15:53:01 UTC 2003
Chuck Yerkes wrote:
> Softupdates (via Kirk McKusick) when used with Sendmail (led by
> Eric Allman) follow the RFCs devoutly. Kirk knows plenty about
> sendmail and Eric knows plenty about file systems. The messages
> are safe and crash proof (barring media corruption).
No, this is not the case. Delayed writes with softupdates guarantee
metadata consistency and dependencies are committed in the proper
order, but a crash could mean that messages -- which appear to
have been accepted by the MTA -- are simply lost.
You need to re-read the softupdates documentation. It's not suitable
for use in implementing a stable queue, even if you make use of the O_DIRECT
flag, because O_DIRECT isn't an ironclad promise:
O_DIRECT may be used to minimize or eliminate the cache effects of read-
ing and writing. The system will attempt to avoid caching the data you
read or write. If it cannot avoid caching the data, it will minimize the
impact the data has on the cache. Use of this flag can drastically
reduce performance if not used with care.
Again, softupdates don't offer a guarantee that the bits and
metadata are committed with a return from write -- if fact, the
performance gains are a result precisely of the fact that it doesn't.
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