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. Thanks. Eugene.
> 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.I too ghave a number of zvols exportes as iscsi, and I was using dev mode util I read this. AM going to experiment with switching back to default mode to see what happens. I also use dev mode for my virtualbox drives. Am going to try and test that too and see what I get. Interesting result though... -pete.
2017-08-30 11:45 GMT+02:00 Eugene M. Zheganin <emz at 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.