On Tuesday 15 June 2010 12:30:50 martin f krafft wrote: > qlso sprach Boyd Stephen Smith Jr. <b...@iguanasuicide.net> [2010.06.15.1840 +0200]: > > > 0 is not a RAID level. > > > > It is a RAID level, now. > > I fail to see how it > has suddenly become a RAID level. ;)
Popular vote. :P ;) > > On the other hand, LVM striping is per-LV. Doing something like > > that with mdadm is... complex. > > Use mdadm for a RAID1 and LVM on top by default. > > Use mdadm for a RAID5 or RAID6 and LVM on top for the remaining > cases when you need space and care less about performance. Use RAID 1/0 in mdadm when you need redundancy, space, and performance. (Although, IME, RAID 5 is not badly performing.) This is particularly useful when you have 3 disks, but only need one disk redundancy. mdadm can layout data like this: | disk1 | disk2 | disk3 | +-------+-------+-------+ | dataA | dataA | dataB | | dataB | dataC | dataC | LVM cannot, easily. RAID 1/0 through mdadm with 4 disks is also better than strictly layering the RAID levels. RAID 0 disks, RAID 1 arrays: | array1 | array2 | | disk1 | disk2 | disk3 | disk4 | +-------+-------+-------+-------+ | dataA | dataB | dataA | dataB | | dataC | dataD | dataA | dataB | | dataE | dataF | dataA | dataB | RAID 1 disks, RAID 0 arrays: | array1 | array2 | | disk1 | disk2 | disk3 | disk4 | +-------+-------+-------+-------+ | dataA | dataA | dataB | dataB | | dataC | dataC | dataD | dataD | | dataE | dataE | dataF | dataF | mdadm 0/1 4 disk, 1 redundant copy of data: | disk1 | disk2 | disk3 | disk4 | +-------+-------+-------+-------+ | dataA | dataA | dataB | dataB | | dataC | dataD | dataC | dataD | | dataE | dataF | dataF | dataE | (same redundancy level as RAID 5, no parity calculations needed.) mdadm 0/1 4 disk, 2 redundant copy of data: | disk1 | disk2 | disk3 | disk4 | +-------+-------+-------+-------+ | dataA | dataA | dataA | dataB | | dataB | dataB | dataC | dataC | | dataC | dataD | dataD | dataD | (same redundancy level as RAID 6, although some capacity may be lost near the end, no parity calculations needed.) > Use LVM without RAID if you need space (and/or performance) and have > the data mirrored elsewhere. I fail to see the advantage of RAID0 in > this scenario, as LVM is more flexible. As long as you don't want to combine redundancy and striping, I agree. When you want to combine them, mdadm's RAID 1/0 is better. Using mdadm to do just RAID 0 is only useful if you have some reason not to want to LVM striping. -- Boyd Stephen Smith Jr. ,= ,-_-. =. b...@iguanasuicide.net ((_/)o o(\_)) ICQ: 514984 YM/AIM: DaTwinkDaddy `-'(. .)`-' http://iguanasuicide.net/ \_/
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