Am 18.02.2017 um 23:20 schrieb Phil Turmel:

On 02/17/2017 05:03 AM, Reindl Harald wrote:

quoted

quoted

Be careful. Don't confuse Raid10 with Raid1+0. They are NOT the
same thing (on linux at least), although they are very similar

yeah, i realized that but anyways thought the "writemostly" logic is
there too and maybe the docs not up-to-date

Linux MD raid10 doesn't have a requirement that the number of devices
be a multiple of the number of data copies.  Which creates "interesting"
data layouts with odd numbers of devices or similar effects with ,n3 or
,f3 layouts.  Which makes it difficult if not impossible to designate
specific devices as write mostly without weird operational asymmetries
across the assembled array.

but since --writemostly doesn't get without manually intervention that 
cases would be unchanged (besides that they are unlikely)

In other words, it is not at all like raid 1 on top of raid 0, except in
certain very limited cases, and your assumptions are simply wrong.

If there are features (other than layouts) of raid10 that make you
prefer it to raid1, it would make sense to ask for those features to
be implemented in raid1.

writemostly it's also very appealing on existing setups, the machine 
from where i type was installed in 2011

RAID1 don't have the benefit of doubled performance (also for writes, on 
a hybrid RAID slower but still faster than RAID1) *and* doubled space 
compared to a single disk combined with mirroring

another example: on machines like a HP microserver with only 4 drive 
slots that you could easily improve read-performance which is for many 
workloads the most important part by just switch half of the disk to SSD

price calculation for a hybrid RAID10 with 10 disks:
5x4 TB SSD = 5 x 1400€ = 7000€
5x4 TB HDD = 5 x 100€ = 500€
total price 7500€ versus 14000€ for flash-only

quoted

sadly i can't write a patch on my own but only point how useful it
would be

That's unfortunate.  Patches are generally welcome

i would be *seriously* willing to pay the inital patch for any kernel 
maintainer who takes it over - Fedora regulary does kernel-rebases on GA 
versions