Re: RAID-10 explicitly defined drive pairs?

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Is there any way how to tell mdadm explicitly how to set up
the pairs of mirrored drives inside a RAID-10 volume?

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If you're using RAID10,n2 (the default layout) then adjacent
pairs of drives in the create command will be mirrors, [ ... ]

I did that once with a pair of MD1000 shelves from Dell and that
worked pretty well (except it was very painful to configure the
shelves with each disk as separate volume).

half of the disks forming the RAID-10 volume disappeared.
After removing them using mdadm --remove, and adding them
back, iostat reports that they are resynced one disk a time,
not all just-added disks in parallel.

That's very interesting news. Thanks for reporting this though,
it is something to keep in mind.

[ ... ] Otherwise it would be better for us to discard RAID-10
altogether, and use several independent RAID-1 volumes joined
together

I suspect that that MD runs one recovery per array at a time,
and 'raid10' arrays are a single array.

Which would be interesting to know in general, for example how
many drives would be rebuilt at the same time in a 2-drive
failure on a RAID6.

You might try a two layer arrangements, as a 'raid0' of 'raid1'
pairs, instead of a 'raid10'. The two things with MD are not the
same, for example you can do layouts like a 3-drive 'raid10'.

using LVM (which we will probably use on top of the RAID-10
volume anyway).

Oh no! LVM is nowhere as nice as MD for RAIDing and is otherwise
largely useless (except regrettably for snapshots) and has some
annoying limitations.