Date: Fri, 19 May 2006 15:29:43 -0400 From: James Snow <snow@teardrop.org> To: "Rick C. Petty" <rick-freebsd@kiwi-computer.com> Cc: freebsd-geom@freebsd.org Subject: Re: Slices + stripes and mirrors Message-ID: <20060519192943.GB54819@teardrop.org> In-Reply-To: <20060519180604.GA37562@megan.kiwi-computer.com> References: <200605161555.08195.darcy@wavefire.com> <20060517171955.GB838@garage.freebsd.pl> <20060519172148.GA54819@teardrop.org> <20060519180604.GA37562@megan.kiwi-computer.com>
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On Fri, May 19, 2006 at 01:06:04PM -0500, Rick C. Petty wrote: > > If you're not mirroring the entire disk, I think you're halfway down a > dangerous path. I think the original poster's use case is a fairly common one. For example, you've got two 40GB disks, and you want to use the first 20GB of each disk for a single 20GB mirror, and the second 20GB for a 40GB stripe. Why does this constitute a dangerous path? > In any case, I've had better luck using gvinum even for just plain > mirroring. I guess we've had somewhat opposite experiences here. I usually wind up digging up old documentation on vinum off the web and fiddling with gvinum until I determine what's still applicable. I found out that I could use 'sd length 0' to autosize disks from this list rather than from any of the docs. Maybe I'm suffering from a severe case of the stupids, but good documentation on gvinum seems to be hard to come by. > IMO, it's easier to use a livecd since you have to reboot the box > anyway. But this assumes you have console access. I've successfully used gmirror to convert machines on the opposite side of the country to RAID without any remote-hands or physical access. It works perfectly for me most of the time. I've little doubt that I'm doing something stupid this time. I should have prefaced my "I'm having some difficulty" comment with "It's working flawlessly for me in six other places. ;) > For that number of steps, why not use gvinum instead? You'd have more > flexibility and you can do things like resize volumes and add/remove > drives while the system is up. You can hot add/remove drives out of a gvinum partition as well. I'll grant that it was somewhat ungraceful, but I did this recently when I needed to duplicate a machine in a hurry. It worked very well. I actually started out playing with gvinum for my setup, but it didn't seem to perform very well. I freely admit I wasn't being very scientific in my testing - just simple dd write tests - but, well, see for yourself: 273k stripe size: 7857123 bytes/sec 8190k stripe size: 8207057 bytes/sec regular disk: 29639806 bytes/sec graid3 disk: 54549442 bytes/sec I was going to try a few different stripe sizes, but the jump between gvinum RAID5 and a single disk was pretty severe. It seemed like more than a stripe size change could remedy. -Snow
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