Date: Thu, 20 Dec 2007 23:17:35 +0100 From: Bernd Walter <ticso@cicely12.cicely.de> To: Peter Schuller <peter.schuller@infidyne.com> Cc: freebsd-fs@freebsd.org, ticso@cicely.de, Ivan Voras <ivoras@freebsd.org> Subject: Re: readv: parallel or sequential? Message-ID: <20071220221735.GB67140@cicely12.cicely.de> In-Reply-To: <200712202140.08367.peter.schuller@infidyne.com> References: <fjbb3v$n60$1@ger.gmane.org> <fjbk9g$pua$1@ger.gmane.org> <20071207174914.GQ10459@cicely12.cicely.de> <200712202140.08367.peter.schuller@infidyne.com>
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On Thu, Dec 20, 2007 at 09:40:00PM +0100, Peter Schuller wrote: > > Yes, but how can it now that it is on a RAID0 and taking advantage of > > multiple spindles instead of making it worse? > > The FS has to do sensible things for single spindle as well. > > And normaly disks are fastest when reading linear and with disk read > > caches this doesn't even have to be interleaved. > > I don't see any potential for parallell access within the same file > > beside some special constructed cases maybe. > > I strongly disagree here. I would expect the operating system to not serialize > all I/O to the same file descriptor/file, if done in paralell from multiple > threads or through AIO. I have at least on use case heavily dependent on > this - does this mean FreeBSD would not be usable for this? No - I just didn't though about this. In most cases a single file means serialized use from the application, but of course this is not true in every case. In case the application uses serialized access there is not much to do beside preread or caching writes to make use of multiple spindles. But an application has to be carefull, because parallel access within a single file almost always mean that access is not linear anymore, so many opther performance tunings won't work as good as they could, so this could easily outweight the performance gain from multiple access. Nonlinear access from within an application has to be for another reason and not as a performance tuning. -- B.Walter http://www.bwct.de http://www.fizon.de bernd@bwct.de info@bwct.de support@fizon.de
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