On Wed, 2006-10-18 at 14:42 +0200, martin f krafft wrote: > Why would anyone want to create a partitionable array and put > partitions in it, rather than creating separate arrays for each > filesystem? Intuitively, this makes way more sense as then the > partitions are independent of each other; one array can fail and the > rest still works -- part of the reason why you partition in the > first place. > > Would anyone help me answer this FAQ? There are a couple reasons I can think. First, not all md types make sense to be split up, aka multipath. For those types, when a disk fails, the *entire* disk is considered to be failed, but with different arrays you won't fail over to the next path until each md array has attempted to access the bad path. This can have obvious bad consequences for certain array types that do automatic failover from one port to another (you can end up getting the array in a loop of switching ports repeatedly to satisfy the fact that one array failed over during a path down, then the path came back up, and another array stayed on the old path because it didn't send any commands during the path down time period). Second, convenience. Assume you have a 6 disk raid5 array. If a disk fails and you are using a partitioned md array, then all the partitions on the disk will already be handled without using that disk. No need to manually fail any still active array members from other arrays. Third, safety. Again with the raid5 array. If you use multiple arrays on a single disk, and that disk fails, but it only failed on one array, then you now need to manually fail that disk from the other arrays before shutting down or hot swapping the disk. Generally speaking, that's not a big deal, but people do occasionally have fat finger syndrome and this is a good opportunity for someone to accidentally fail the wrong disk, and when you then go to remove the disk you create a two disk failure instead of one and now you are in real trouble. Forth, to respond to what you wrote about independent of each other -- part of the reason why you partition. I would argue that's not true. If your goal is to salvage as much use from a failing disk as possible, then OK. But, generally speaking, people that have something of value on their disks don't want to salvage any part of a failing disk, they want that disk gone and replaced immediately. There simply is little to no value in an already malfunctioning disk. They're too cheap and the data stored on them too valuable to risk loosing something in an effort to further utilize broken hardware. This of course is written with the understanding that the latest md raid code will do read error rewrites to compensate for minor disk issues, so anything that will throw a disk out of an array is more than just a minor sector glitch. > (btw: [0] and [1] are obviously for public consumption; they are > available under the terms of the artistic licence 2.0) > > 0. http://svn.debian.org/wsvn/pkg-mdadm/mdadm/trunk/debian/FAQ?op=file&rev=0&sc=0 > 1. http://svn.debian.org/wsvn/pkg-mdadm/mdadm/trunk/debian/README.recipes?op=file&rev=0&sc=0 > -- Doug Ledford <dledford@xxxxxxxxxx> GPG KeyID: CFBFF194 http://people.redhat.com/dledford Infiniband specific RPMs available at http://people.redhat.com/dledford/Infiniband
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