Every block has it's copy (') on the other drive at the same position.
With the normal "near" copy layout the RAID10 personality would order the blocks in the same way. However, with the "far" copy layout, both drives are divided in 2 parts. The copy of each block is still located on the other drive, but at the other half of the drive.
The effect is still the same, each drive has all the blocks, so a drive can still fail without impact on the data. However, the interesting part is that the first half of the drive is now ordered as a stripe. That means there should be an improvement when reading data in this configuration. Writing is different though. There should be a lot more seeks to place the data on the drives, alternating between the two halves of the drive.
Just to see how it holds up in real live I did a small test on my server with 2 spare partitions on 2 different drives. First I setup a standard RAID1 with a chuncksize of 64kb and made an ext3 filesystem.
Then I ran bonnie++ on it:
Then destroy the RAID1 and make a RAID10 with far copies.
And again runing bonnie++:
So judging from the results the increase in read performance is there. On this size of partition there seems to be no big difference in write performance. However, I can imagine this getting worse when the partition size increases. Still, this is an interesting setup to look at, especially for system drives (excluding the boot partition ofcourse, unless you have a bootloader which supports this).
