Date: Wed, 10 Aug 2016 13:44:04 +0200 From: Edward Tomasz =?utf-8?Q?Napiera=C5=82a?= <trasz@FreeBSD.org> To: Ben RUBSON <ben.rubson@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: [iSCSI] Trying to reach max disk throughput Message-ID: <20160810114404.GA80485@brick> In-Reply-To: <6B32251D-49B4-4E61-A5E8-08013B15C82B@gmail.com> References: <6B32251D-49B4-4E61-A5E8-08013B15C82B@gmail.com>
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On 0810T1154, Ben RUBSON wrote: > Hello, > > I'm facing something strange with iSCSI, I can't manage to reach the expected disk throughput using one (read or write) thread. [..] > ### Initiator : iscsi disk throughput : > > ## dd if=/dev/da8 of=/dev/null bs=$((128*1024)) count=81920 > 10737418240 bytes transferred in 34.731815 secs (309152234 bytes/sec) - 295MB/s > > With 2 parallel dd jobs : 345MB/s > With 4 parallel dd jobs : 502MB/s > > > > ### Questions : > > Why such a difference ? > Where are the 167MB/s (462-295) lost ? Network delays, I suppose. A single dd(1) would spend some time waiting for the data to get pushed over the network - due to delays (lag), not bandwidth. Having multiple ones makes it possible to compensate, by having multiple outstanding IO operations.
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