Am 19.02.2017 um 18:31 schrieb Phil Turmel:

On 02/18/2017 06:35 PM, Reindl Harald wrote:

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Am 18.02.2017 um 23:20 schrieb Phil Turmel:

quoted

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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

Doubled capacity?  Vs. raid1?  No.  Raid10,n2 (,n2 is default) on two
devices yields the same capacity as raid1 on two devices.  Unless I'm
misunderstanding your point.

you are misunderstanding

RAID1:  2x2 TB = 2 TB usable
RAID10: 4x2 TB = 4 TB useable

typically smaller disks are cheaper and when i installed the 4x2 TB 
RAID10 4 TB disks where not that common and 4 TB SSD not available at 
all (and 2 TB SSD unpaibale)

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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

What is preventing you from using the existing raid1 in pairs with
write mostly, then layering raid0 on top of them for the capacity you
are trying to achieve?  No new code required.  What you are asking for
really is raid1+0, which MD raid allows you to assemble yourself.

already existing setups and the easier configuraion of RAID10 than wrap 
2 RAID1 into a RAID0 especially at inital setup time when you also cover 
the os setup itself

/dev/md0         ext4        485M     33M  448M    7% /boot
/dev/md1         ext4         29G    6,8G   22G   24% /
/dev/md2         ext4        3,6T    2,3T  1,4T   63% /mnt/data

md0: RAID1
md1: RAID10
md2: RAID10

it's really not funny to change that existing layout from RAID10 to 
RAID0+RAID1

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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

Since no new kernel code is needed to achieve what you desire, I doubt
a kernel patch for it would be accepted. (But I'm not a maintainer, so
YMMV.)  This is really a user-space question, along the lines of
"should/could mdadm automate creation of dual layers like raid1+0?"

at least "mdadm" in the current state should just refuse 
"--write-mostly" when the array is a RAID10 - in that case i would have 
known by testing it based on http://www.tansi.org/hybrid/ in a virtual 
machine that it *really* don't work with RAID10

obviously there is code needed to achieve "writemostly" on the most 
common setup of 4 disks for a RAID10 where you later try to replace half 
of the disks with SSD and have writes only on the remaining HDD

there are so many workloads where read-performance is more imprtant 
(boot, start of large applications, start virtual machines, rsync large 
data...)