On Fri, Sep 5, 2014 at 9:01 AM, Warren Young warren@etr-usa.com wrote:
So the real question is, why do you believe you need to make each RAID member a *partition* on a disk, instead of just take over the entire disk? Unless you're going to do something insane like:
/dev/md0 /dev/sda1 /dev/sdb1 /dev/md1 /dev/sda2 /dev/sdb2...you're not going to get any direct utility from composing a RAID from partitions on the RAID member drives.
(Why "insane?" Because now any I/O to /dev/md1 interferes with I/O to /dev/md0, because you only have two head assemblies, so you've wiped out the speed advantages you get from RAID-0 or -1.)
Well, to exactly the same extent that putting multiple partitions and filesystems on a non-raid drive is insane for those reasons... And you generally can't avoid this if you want to boot from the same disks where you store data with mirroring. And the very nice up side is that you can now pull your drives out, put them in different bays, add others, etc. and the system will still assemble the right partitions into the right raid devices and mount them correctly. Or at least it would in the < 2TB days...
There are ancillary benefits, like the fact that a RAID element that spans the entire partition is inherently 4k-aligned. When there is a partition table taking space at the start of the first cylinder, you have to leave the rest of that cylinder unused in order to get back into 4k alignment.
Isn't it possible to duplicate that when you make a single partition and use the partition as a raid member? And get autoassembly if it is less than 2TB? I consider it a real loss that autoassembly doesn't work on large drives. People will almost certainly lose data in some scenarios as a result.
The only downside I saw in this thread is that when you pull such a disk out of a Linux software RAID and put it into another machine, you don't see a clear Linux partition table, so you might think it is an empty drive. But the same thing is true of a hardware RAID member, too.
I've always liked software raid1 just because you can recover the data from any single drive on any machine with a similar interface. But, I guess that's why we have backups...