Date: Thu, 21 Sep 2006 14:52:38 -0400 From: John Nielsen <lists@jnielsen.net> To: freebsd-questions@freebsd.org Subject: Re: geom - help ... Message-ID: <200609211452.39110.lists@jnielsen.net> In-Reply-To: <45122531.6010503@infracaninophile.co.uk> References: <20060920222944.M1031@ganymede.hub.org> <45122531.6010503@infracaninophile.co.uk>
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On Thursday 21 September 2006 01:37, Matthew Seaman wrote: > Marc G. Fournier wrote: > > So, again, if I'm reading through things correctly, I'll have to do > > something like: > > > > gstripe st1 da1 da2 > > gstripe st2 da3 da4 > > gmirror drive st1 st2 > > newfs drive > > That's the wrong way round, I think. If you lose a drive, then you've > the whole of one of your stripes and have no resilience. Shouldn't you > rather stripe the mirrors: > > gmirror gm0 da1 da2 > gmirror gm1 da3 da4 > gstripe gs0 gm0 gm1 > newfs gs0 > > This way if you lose a drive then only one of your gmirrors loses > resilience and the other half of your disk space is unaffected. I would recommend the 1+0 approach as well. In addition to increasing your= =20 odds of surviving a multi-disk failure, it makes replacing a failed compone= nt=20 easier and faster--you only need to rebuild component mirror (which involve= s=20 one command and duplication of half of the total volume) instead of=20 recreating a component stripe and then rebuilding the whole mirror (which=20 involves at least two commands and duplication of the entire volume). Regarding the spare, I think you're right that there isn't (yet) a way to=20 configure a system-wide hot spare, but it would not be hard to write a=20 monitoring script that gives you essentially the same thing. Assuming the 1= +0=20 approach: every N seconds, check the health of both mirrors (using "gmirror= =20 status" or similar). If volume V is degraded, do a "gmirror forget V; gmirr= or=20 insert V sparedev", e-mail the administrator, and mark the spare as=20 unavailable. After the failed drive is replaced, the script (or better, a=20 knob that the script knows how to check) should be updated with the=20 devicename of the new spare. =46or a 50% chance of having zero time-to-recovery (at the cost of more=20 expensive writes), you could also add the spare as a third member to one of= =20 the mirror sets. If a member of that set fails, you still have a redundant= =20 mirror. If a member of the other set fails, you just do a "gmirror remove" = to=20 free the spare from the 3-way mirror and then add it to the failed set. =46rom my own experience, I've been very happy with both gmirror and gstrip= e,=20 and in fact I just finished setting up a rather unorthodox volume on my=20 desktop at work. I have three drives (two of which were scavenged from othe= r=20 machines): one 60GB and two 40GB. I wanted fault tolerance for both /=20 and /usr, I wanted /usr to be as big as possible, and I wanted reasonable=20 performance. I ruled out graid3 and gvinum raid5 since I want to be able to= =20 boot easily from / and performance would be poor since the 40GB drives shar= e=20 a controller. I made / a mirror of two 10GB partitions on the 40GB drives,= =20 made a stripe out of the remaining 30GB from the 40GB drives, and added the= =20 stripe into a mirror set with the 60GB drive. It's working quite nicely so= =20 far. JN
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