Date: Wed, 27 Sep 2017 13:07:38 +0200 From: Edward Napierala <trasz@freebsd.org> To: "Eugene M. Zheganin" <emz@norma.perm.ru> Cc: FreeBSD Stable <freebsd-stable@freebsd.org> Subject: Re: zfs, iSCSI and volmode=dev Message-ID: <CAFLM3-pyh4fbGKbM5Y=26RUFc9ozsxobuwm9ayPkk3hwp0uWig@mail.gmail.com> In-Reply-To: <ece540ba-8039-424f-8bad-c4266d70d84b@norma.perm.ru> References: <ece540ba-8039-424f-8bad-c4266d70d84b@norma.perm.ru>
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2017-08-30 11:45 GMT+02:00 Eugene M. Zheganin <emz@norma.perm.ru>: > Hi, > > > I have an iSCSI production system that exports a large number of zvols as > the iSCSI targets. System is running FreeBSD 11.0-RELEASE-p7 and initially > all of the zvols were confugured with default volmode. I've read that it's > recommended to use them in dev mode, so the system isn't bothered with all > of these geom structures, so I've switched all of the zvols to dev mode, > then I exported/imported the pools back. Surprisingly, the performance has > fallen down like 10 times (200-300 Mbits/sec against 3-4 Gbits/sec > previously). After observing for 5 minutes the ESXes trying to boot up, and > doing this extremely slowly, I switched the volmode back to default, then > again exported/imported the pools. The performance went back to normal. > > > So... why did this happen ? The result seems to be counter-intuitive. At > least not obvious to me. I don't really have an answer - mav@ would be the best person to ask. Based on his description, "ZVOLs in GEOM mode don't support DPO/FUA cache control bits, had to chunk large I/Os into MAXPHYS-sized pieces and go through GEOM." There also used to be so that TRIM was only supported in the "dev" mode, but that changed a while ago.
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