Re: POSIX violation by writeback error

Rogier Wolff
2018-09-05T08:04:49Z

On Wed, Sep 05, 2018 at 09:39:58AM +0200, Martin Steigerwald wrote:
> Rogier Wolff - 05.09.18, 09:08:
> > So when a mail queuer puts mail the mailq files and the mail processor
> > can get them out of there intact, nobody is going to notice. (I know
> > mail queuers should call fsync and report errors when that fails, but
> > there are bound to be applications where calling fsync is not
> > appropriate (*))
>
> AFAIK at least Postfix MDA only reports mail as being accepted over SMTP
> once fsync() on the mail file completed successfully. And I´d expect
> every sensible MDA to do this. I don´t know how Dovecot MDA which I
> currently use for sieve support does this tough.

Yes. That's why I added the remark that mailers will call fsync and know
about it on the write side. I encountered a situation in the last few
days that when a developer runs into this while developing, would have
caused him to write:
/* Calling this fsync causes unacceptable performance */
// fsync (fd);

I know of an application somewhere that does realtime-gathering of
call-records (number X called Y for Z seconds). They come in from a
variety of sources, get de-duplicated standardized and written to
files. Then different output modules push the data to the different
consumers within the company. Billing among them.

Now getting old data there would be pretty bad. And calling fsync
all the time might have performance issues....

That's the situation where "old data is really bad".

But when apt-get upgrade replaces your /bin/sh and gets a write error
returning error on subsequent reads is really bad.

It is more difficult than you think.

Roger.

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