Date: Wed, 14 Aug 2013 17:29:13 +0200 From: Willem Jan Withagen <wjw@digiware.nl> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: fs@freebsd.org Subject: Re: Fwd: Disk scheduling activity... Message-ID: <520BA249.8030603@digiware.nl> In-Reply-To: <alpine.GSO.2.01.1308140859070.2267@freddy.simplesystems.org> References: <520B8B1E.7060002@digiware.nl> <alpine.GSO.2.01.1308140859070.2267@freddy.simplesystems.org>
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On 2013-08-14 16:03, Bob Friesenhahn wrote: > On Wed, 14 Aug 2013, Willem Jan Withagen wrote: >> >> Just a point of information or curiosity, and I don't think/know if it >> is any problem... >> >> I have the raidz array with 8 disks, which I'm using to backup to. >> It is configured >> 4 disks on a mvs controller >> 4 disks on an Areca controller (JBODs with battery) >> Both controllers are on a PCI-E slot >> >> Most of the time the source just fully loads the pipe and sends 1Gbit/s. >> >> When that happens I see this alternating pattern of writing either to >> the 4 mvs disks, or writing to the Areca disks. >> But almost never are all disk accesses at the same time. >> And really never, never is there a mix of writing between the controller >> sets. > > Are all 8 disks in the same raidz vdev? Yes is a raidz1 with 8 disks. I know it is not optimal in performance, but I needed the amount of remaining diskspace. > Are you basing write activity on the drive LEDs? Yup. > > The Areca controller may be caching the writes in its battery-backed > cache and deferring the writes to when zfs tells it to flush its cache. > The other controller may be issuing the writes right away. This would > explain apparent 'split' writing behavior. Sounds like a fair assumption. Could remove the battery and see what happens then. The mvs device is relatively "simple" and has no significant memory on board. > There is even the possibilty that one of the controllers ignores the > cache flush request and performs the writes later when it feels like it. That would then be the Areca controller, bacause I have the feeling that it always writes later. --WjW
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