[CentOS] RAID MD10

Mon Mar 4 16:54:07 UTC 2013
Markus Falb <wnefal at gmail.com>

On 04.Mär.2013, at 17:39, m.roth at 5-cent.us wrote:

> zGreenfelder wrote:
>> On Mon, Mar 4, 2013 at 11:20 AM, John Plemons <john at mavin.com> wrote:
>>> Raid 10 is a mirrored stripped set of at least 4 driver. You get the
>>> best of both worlds, data speed and data back up..
>> yeah, that's the industry standard.   he's asking you to go find and read
>> http://en.wikipedia.org/wiki/Raid10#Near_versus_far.2C_advantages_for_bootable_RAID
>> wherein they mention that linux md devices can do what they call a
>> raid 10 on 2 drives. and then details some of the reasons you might
>> want to do such a thing.
>> I can't see any reason to go with the sorta raid 10 on only 2 drives.
>> from that article, I'd the only sane choice for raid 10 on 2 drives
>> is the 'far' config on SSD drives.   but that's just my opinion.   I
>> don't think I'd ever pick raid10 on 2.
>> from the entry:
>> "...copies of a block of data are "near" each other or at the same
>> address on different devices or predictably offset: Each disk access
>> is split into full-speed disk accesses to different drives, yielding
>> read and write performance like RAID 0 but without necessarily
>> guaranteeing that every stripe is on both drives"
>> which then some (and by murphy's rule will be the most critcal) will
>> go from being raid 10 to raid0.  and likely 0 on the drive that fails.
> AHHH! I didn't read closely enough, and missed that lack of guarantee.
> Thanks, *that's* the kind of discussion I was looking for.

Note that you can do 2 copies to 3 disks, or 3 copies to 4 disks, …
Of course not every stripe is on *every* disk in that case.
If you have 2 copies, one disk may fail fail. If you have 2 copies on 2 disks, 1 disk may fail. That's how I read it.

Kind Regards, Markus