{upstream} missing command to start *single* arrays in "auto-read" mode (like --incremental)

[ceg]

Binary package hint: mdadm

Proposed command: "mdadm --incremental --run /dev/mdX"

Initramfs must be able to start (selected) md devices in degraded mode (only the ones that contain the rootfs and swap/resume partition), if they didn't start after some time of udev hotplugging. Starting all arrays, that are not yet complete at that early stage, degraded is not acceptable.

Initramfs could use the old non-incremental commands,

( i.e. "mdadm --run /dev/mdX" is preferable to "mdadm --assemble --scan" to start only selected array degraded)

but the old commands break the support for "auto-read" mode set up and smooth inclusion of members that appear later on.

Alternatively to a new --incremental option, make the old commands compatible and also start degraded arrays in "auto-read" mode and incrementally extendable by default.

[ceg (ceg)]

can not change this to wishlist / upstream

[ceg (ceg)]

Current state of ubuntu systems with md raid: https://wiki.ubuntu.com/ReliableRaid

[ceg (ceg)]

workaround sequence:

mdadm --remove <incomplete-md-device> <arbitrary-member-device-of-incomplete-array>

mdadm --incremental --run <arbitrary-member-device-of-incomplete-array>

[ceg (ceg)]

Are you aware that starting all arrays degraded (what the initramfs scripts currently have to do) is a serious bug? The replicas will diverge, and if the non-rootfs devices become accessible later they need to resync.

[Dimitri John Ledkov (xnox)]

On 21/05/12 17:32, ceg wrote:
> Are you aware that starting all arrays degraded (what the initramfs
> scripts currently have to do) is a serious bug? The replicas will
> diverge, and if the non-rootfs devices become accessible later they need
> to resync.
>

Yes, I am aware. That's bug #497186 and it's on my radar to fix for
Quantal as part of
https://blueprints.launchpad.net/ubuntu/+spec/foundations-q-software-raid

[ceg (ceg)]

All right, hopefully some of the stuff I filed is of help to you, thanks!

[Dimitri John Ledkov (xnox)]

Isn't this fixed with new mdadm having these modes:

mdadm --incremental --run

&

mdadm --incremental --scan --run

?????

[ceg (ceg)]

I don't think so.

My guess is that if initramfs calls

mdadm --incremental --run

it would run all arrays and not just the single array (mdX) that contains the root filesystem.
Degrading not yet fully available non-root arrays is bad, as it may nullify the redundancy and take hours to get the arrays resynced again.