also sprach Boyd Stephen Smith Jr. <bss@iguanasuicide.net> [2010.06.15.2108 +0200]:
> > 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.)
Sure it is, on writes. If you have the right hardware, you won't
notice, but every write takes twice as much work.
http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob;f=docs/RAID5_versus_RAID10.txt;hb=refs/heads/contrib/docs/raid5-vs-raid10
> 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.
Do you have any data to back this up? Fundamentally, the data will
traverse one layer less, but does it actually make a difference?
> 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.)
More info (and patches welcome):
http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob;f=docs/RAID5_versus_RAID10.txt;hb=refs/heads/contrib/docs/raid5-vs-raid10
http://git.debian.org/?p=pkg-mdadm/mdadm.git;a=blob;f=debian/FAQ;hb=HEAD#l106
--
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