Re: [PATCH} Add RAID devices.

[Chris Marusich <cmmarusich@gmail.com>]

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Hi Andreas,

Ludo's response clarified a lot of things for me.  The only remaining
feedback I have is that (1) to aid the reader, you should consider
adding a cross-reference from "(guix) Mapped Devices" (in the part where
you mention that certain modules must be added) to "(guix) Initial RAM
Disk", and (2) you might want to look into using the "Auto Assembly"
feature of mdadm (see below).

If you've tested the changes, though, and it works, I see no reason not
to commit this and enable everyone to enjoy the use of RAID arrays! :)

Andreas Enge <andreas@enge.fr> writes:

> On Sat, Jul 23, 2016 at 10:43:58PM -0700, Chris Marusich wrote:
>> Cool!  Is it possible to use them in combination?  Using the example
>> From the documentation, would it possible to use LUKS to create an
>> encrypted /dev/mapper/home which uses /dev/md0 instead of /dev/sda3?
>
> unfortunately not. This is an area where some work should be invested.
> It would require to do things properly in order also. LVM support also
> comes to mind. I think these devices can be staged in an arbitrarily
> complex way, no? Encrypting a RAID device, or creating a RAID device
> from encrypted partitions, for instance (of which the former sounds more
> reasonable to me).

I agree it would be a nice feature, but if your patch is working right
now to enable the use of RAID, then I think it would be fine to submit
your patch now and add such a feature later.

>> I understand that in Linux, the device file names (e.g., /dev/sda3) can
>> sometimes change unexpectedly.  If they change later on, will anything
>> bad happen?
>
> Yes, the RAID could not be assembled, and then the machine could not boot
> up. But I have never experienced this kind of problem for hard disks.

It occurs occasionally [1], but the fewer disks you have, the less
likely you are to observe it.  I wonder if you can use mdadm's "Auto
Assembly" feature (see "man 8 mdadm" for details) to avoid this issue
entirely?  It sounds like you might not even need to specify a source
device list, if the description of "Auto Assembly" is to be believed.

[1] For example: https://serverfault.com/questions/140071/hard-drive-device-names-are-different-from-one-reboot-to-another-in-ubuntu

-- 
Chris

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