Date: Thu, 28 Nov 2013 15:15:11 -0700 (MST) From: Warren Block <wblock@wonkity.com> To: =?ISO-8859-15?Q?Szalai_Andr=E1s?= <szalai.bandi@gmail.com>, Schaich Alonso <alonsoschaich@fastmail.fm> Cc: freebsd-stable@freebsd.org Subject: Re: gmirror: writes are faster than reads Message-ID: <alpine.BSF.2.00.1311281512170.50600@wonkity.com> In-Reply-To: <20131128220305.e715adb95b16f494224052f5@fastmail.fm> References: <5297ABD5.5060504@gmail.com> <20131128220305.e715adb95b16f494224052f5@fastmail.fm>
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On Thu, 28 Nov 2013, Schaich Alonso wrote: > Modern HDDs have both Command Queuing and Excessive Cache Memory. Using them > a spindle disk can cache multiple write requests and do them all in one > revolution. While multiple read requests can also be done at once chances > are the on-disk cache is not usefull (because the requested data is only > resident if it was accessed short before, and then it's also availible in > the kernel's larger Filesystem/GEOM caches which reside in main memory and > were consulted prior to disk accesses) and the GEOM layer might not have > issued them yet. IIRC the UFS subsystem will perform no read requests smaller > than 512kB by default, which means it does some readahead just in case the > issuing application wants to read more data soon - however you have used read > blocks which are exact multiples of 512kB, so there is no gain in this. > > readahead is the buzzword for tuning large sequencial reads, and I had thought > there was a sysctl for it, though I can't find it now. gmirror also has four different load-balancing algorithms which can affect read and write speeds.
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