Date: Wed, 25 Oct 2006 23:11:53 +0200 From: Guido van Rooij <guido@gvr.org> To: Oliver Fromme <olli@lurza.secnetix.de> Cc: freebsd-stable@FreeBSD.ORG Subject: Re: Gmirror performanc Message-ID: <20061025211153.GA58556@gvr.gvr.org> In-Reply-To: <200610251350.k9PDo442086250@lurza.secnetix.de> References: <20061025132455.GA52157@gvr.gvr.org> <200610251350.k9PDo442086250@lurza.secnetix.de>
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On Wed, Oct 25, 2006 at 03:50:04PM +0200, Oliver Fromme wrote: > Guido van Rooij wrote: > > Anyway, I created a gm device and a partition. Now the read performance > > is not what I'd expect. > > I have the partition on two SATA devices on different controlers. > > I get around 60MB/s for each disk. I can get that speed from both disks > > simultaneously. > > Now when I dd from the gm device, I don't get any speed higher than that. > > That's expected. I do not agree. > > > I tried with -b split -s <various sizes>, -b round-robin, -b load. > > (dd-ing as done with a bs of 1m; I see the transaction size is 128Kb, > > unless the split method is used, in which case the transaction size > > gies down. When round-robin is used, the transaction size is 128Kb/s, > > but the number of transaction per second goes down.). > > > > I cannot explain why I should not get a higher read speed. Anyone? > > dd is a sequential, single-threaded operation, so it will > only use one disk at a time. It's not really suitable as > a benchmark for real-world things. I never argued that. > > In the real world [TM] you have multiple processes that > access the file system at random. Here you will benefit > from the mirror, because accesses will be distributed > among the disks. But a single-threaded process accessing a large file should also be able to see a speed increase. I really do not see why a split or roundrobin approach to a mirror would get only half the performance of a raw access to a non-mirror. Somehow there must a limit of 128KB of outstanding requests. That limit is software imposed and should IMHO scale with the number of spindles in a mirror... -Guido that can be
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