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SubjectRe: O_SYNC: How well do we support it?
Hi,

On Fri, 07 Jan 2000 09:47:54 -0500, Peter Rival <frival@zk3.dec.com>
said:

> Yup, I was only looking at the O_SYNC code, as that's the only sync
> work that AIM does. I'll take a peek at the fdatasync() stuff as
> well. BTW, I believe that the O_SYNC flag also requires that (for
> strict POSIX compliance) we write out the updated metadata (mtime,
> etc.) synchronously as well as the data. However, I also know that
> some operating systems don't do this unless specifically requested by
> the user due to the additional performance overhead.

All updated metadata is always by O_SYNC/fsync, with the _only_
exception of the timestamps. Strictly, even timestamp flush is required
by POSIX, but it's just too slow in most cases. In POSIX, O_DATASYNC
and fdatasync() are explicitly allowed to omit the timestamp updates if
that is the only dirty metadata involved.

> Should we add in the capacity to perform synchronous metadata updating
> as well?

We have to, there's no choice about metadata in general. timestamps are
a special case where it is legitimate to consider relaxing the flush
rule for the fdatasync variants, or even for fsync if specified by a
sysctl parameter (which is what at least one other Unix does).

--Stephen

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