Re: which OutputStreams are buffered?

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Tue, 20 May 2008 21:44:29 -0400
Tom Anderson wrote:

Oh christ - i just looked up what the Open Group have to say about it
[1], and according to IEEE Std 1003.1, 2004 Edition, aka POSIX:

"The sync() function shall cause all information in memory that updates
file systems to be scheduled for writing out to all file systems.

"The writing, although scheduled, is not necessarily complete upon
return from sync()."

Although that's the all-files sync, and not the just-this-file fsync,
which says:

"The fsync() function shall request that all data for the open file
descriptor named by fildes is to be transferred to the storage device
associated with the file described by fildes. The nature of the transfer
is implementation-defined. The fsync() function shall not return until
the system has completed that action or until an error is detected."

Which sounds a bit sketchy, but basically what we want. But then it
comes back with:

"If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on
the conformance document to tell the user what can be expected from the
system. It is explicitly intended that a null implementation is permitted."

At that level there are not much guarantees for anything.

I guess one of the reasons is that it can be very difficult to
implement an API that make it 100% sure the data is at location that
is rotating. Cache in RAID controllers, cache in disk drives,
file servers, NAS, SAN etc.etc..


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