Re: [RFC] relaxed barrier semantics

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On Thu, Jul 29, 2010 at 07:04:06PM -0400, Ted Ts'o wrote:
> My understanding is that "everything FUA" can be a performance
> disaster.  That's because it bypasses the track buffer, and things get
> written directly to disk.  So there is no possibility to reorder
> buffers so that they get written in one disk rotation.  Depending on
> the disk, it might even be that if you send N sequential sectors all
> tagged with FUA, it could be slower than sending the N sectors
> followed by a cache flush or SYNCHRONIZE_CACHE command.

Not sure why the discussion is drifting in this direction again, but no
one suggested to switch eweryone to forcefully use a FUA only primitive.
If we offer a WRITE_FUA primitive to those who can make use of it, it
won't mean the the cache flush primitive will go away - we will need it
to implement fsync anyway.

> 
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