Date: Sat, 14 Nov 2020 12:22:36 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Alexander Motin <mav@FreeBSD.org>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org> Subject: Re: MAXPHYS bump for FreeBSD 13 Message-ID: <07a4ca53-da9d-e7b2-9af3-c5098f15a5c7@selasky.org> In-Reply-To: <1bff381f-3d6e-b20c-28f9-1403a9dfe0f6@FreeBSD.org> References: <1bff381f-3d6e-b20c-28f9-1403a9dfe0f6@FreeBSD.org>
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On 11/14/20 5:14 AM, Alexander Motin wrote: >> We currently have a MAXPHYS of 128k. This is the maximum size of I/Os >> that we normally use (though there are exceptions). >> >> I'd like to propose that we bump MAXPHYS to 1MB, as well as bumping >> DFLTPHYS to 1MB. > > I am all for the MAXPHYS change, as Warner told it was my proposition on > a chat. ZFS uses blocks and aggregates I/O up to 1MB already and can > more potentially, and having I/O size lower then this just overflows > disk queues, increases processing overheads, complicates scheduling and > in some cases causes starvation. > > I'd just like to note that DFLTPHYS should probably not be changed that > straight (if at all), since it is used as a fallback for legacy code. > If it is used for anything else -- that should be reviewed and probably > migrated to some other constant(s). > Beware that many USB 2.0 devices will break if you try to transfer more than 64K. Buggy SCSI implementations! --HPS
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