'resync=PENDING', 'auto-read-only' on raid1 creation

List, good afternoon,

I've recently needed to create a new raid1 array in an existing (non-raid) Debian Wheezy system. The array has been created read-only, and resync is PENDING; I didn't expect either of these conditions.

Objective is to have an LVM built initially on mirrored unused 2TB drives, with a view to growing the LVM with mirrored 3TB drives in the future. The Logical volume will be exported over NFS, and will serve as a repository for backups of other machines. The approach I am following is, first, to create a raid1 array then, second, install an LVM over that array, making sure that I create the LVM such that it can be expanded.

(I think) I've created a raid1 on two empty and unpartitioned 2TB drives, but am not seeing the results that the wiki (https://raid.wiki.kernel.org/index.php/RAID_setup) seems to suggest.

# cat /proc/mdstat
reports

md0 : active (auto-read-only) raid1 sdc[1] sdb [0]' 1953383360 blocks super 1.2 [2/2] [UU] resync=PENDING

This was the create command I used:

# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc

I've read through man mdadm but had not expected 'auto-read-only', nor 'resync=PENDING'. Has something gone wrong, for some reason? Or do I need, simply, to set the array to be read-write? I wondered whether 'create' does not 'start' the array, but it's reported as 'active', so I think the array is started. I have not yet put a filesystem on the array, because I am not sure the creation is ok.

Grateful for any comment, and apologies for what must seem a very basic question,

regards, Ron

Hi Ron,

"auto-read-only" is a safety measure that prevents any writes to an array until something in the data area is written. I don't remember the rules MD uses to determine if any given assemble/create operation qualifies, but it isn't an error.

Just perform pvcreate on the array as you intended, and that'll switch the array to rw (and start the resync).

HTH,

Phil

That explains an issue I had when creating a brand new raid6 and it failed to start a sync as I expected it to... I probably went round the house to "fix" it and ended up using a "missing" then adding the missing drive which started the sync.

I guess its one of the problems, that isn't, that "just happens" when you start to keep too close an eye on things, such as a "watch proc/mdstat" in another window where as before I probably would have just created it, added a file system, and been none the wiser :-/

Yep, it did, and has done; thank you for the re-assurance, Ron