Date: Sun, 24 Nov 2002 04:12:50 +0100 From: Brad Knowles <brad.knowles@skynet.be> To: drew-dated-1038529607.6fb7a6@poured.net Cc: freebsd-chat@freebsd.org, Drew Raines <drew-public@poured.net> Subject: Re: mail queues on softdep (was Re: Sharing calendars?) Message-ID: <a05200f27ba05f1b91a62@[192.168.0.3]> In-Reply-To: <l6vn0nzvlup.fsf_-_@williams.mc.vanderbilt.edu> References: <GEEGJMKEOCMNOBOAHIOMMENACBAA.pcable@slaudiovis.org> <a05200f1bba044cddeec9@[192.168.0.3]> <l6vzns0uvol.fsf@williams.mc.vanderbilt.edu> <3DE00781.F47E59A1@mindspring.com> <l6vn0nzvlup.fsf_-_@williams.mc.vanderbilt.edu>
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At 6:43 PM -0600 2002/11/23, Drew Raines wrote:
>> Why are soft updates bad for mail queues, in your opinions?
>
> My opinions aren't authoritative because I've never (knowingly)
> experienced it, but apparently a message could be lost if it's
> accepted by the MTA and there's a system failure before the
> metadata gets written to the disk.
Softupdates gives us the ability to optimize out certain types of
very fast file creation/deletion pairs, such as happens with mail
messages being in the queue for only the briefest moments of time
between our accepting the message and our successfully delivering the
message to the next hop. Yes, softupdates does slightly increase the
probability of loss of individual messages, if there should be a
crash at a critical time.
However, for those MTAs which take care to make sure that they
perform certain file operations in a certain order (e.g., sendmail &
postfix), this is generally not considered to be a violation of RFC
2821, section 6.1:
6.1 Reliable Delivery and Replies by Email
When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
message in response to DATA), it is accepting responsibility for
delivering or relaying the message. It must take this responsibility
seriously. It MUST NOT lose the message for frivolous reasons, such
as because the host later crashes or because of a predictable
resource shortage.
For those MTAs which do not take care to perform those file
operations in a certain order (e.g., qmail & exim), this is not true.
They are inherently incompatible with softupdates, and in order to
comply with this section of the RFC, you must either not use them, or
you must turn off softupdates.
> This could happen with sync, too, I guess, but the latency is
> increased with soft updates. There might be characteristics of
> qmail's queue which causes it to be more susceptible than others,
> but the logic seems to apply to all.
This is a specific problem with qmail and exim. This is not a
general problem with softupdates as a whole.
--
Brad Knowles, <brad.knowles@skynet.be>
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
-Benjamin Franklin, Historical Review of Pennsylvania.
GCS/IT d+(-) s:+(++)>: a C++(+++)$ UMBSHI++++$ P+>++ L+ !E W+++(--) N+ !w---
O- M++ V PS++(+++) PE- Y+(++) PGP>+++ t+(+++) 5++(+++) X++(+++) R+(+++)
tv+(+++) b+(++++) DI+(++++) D+(++) G+(++++) e++>++++ h--- r---(+++)* z(+++)
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