Date: Wed, 14 Aug 2013 10:36:12 -0500 From: Karl Denninger <karl@denninger.net> Cc: fs@freebsd.org Subject: Re: Fwd: Disk scheduling activity... Message-ID: <520BA3EC.1030304@denninger.net> In-Reply-To: <520BA249.8030603@digiware.nl> References: <520B8B1E.7060002@digiware.nl> <alpine.GSO.2.01.1308140859070.2267@freddy.simplesystems.org> <520BA249.8030603@digiware.nl>
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On 8/14/2013 10:29 AM, Willem Jan Withagen wrote: > 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 > _ I very much doubt the ARECA is ignoring the cache-flush request. I have several of these and can get them into a pathological state with TERRIBLE performance when ZFS starts doing things that demand cache flushes - the ARECA will perform the demanded flush which, if you have a lot of RAM on the board, gets real interesting in terms of performance impact. -- Karl Denninger karl@denninger.net /Cuda Systems LLC/
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