[Qemu-discuss] OK to mdadm --assemble same array simultaneously in both host and guest?

[Frank Myhr]

Hi,

I have a temporary situation where I'm passing entire physical drives into a
Linux guest as raw block devices and using mdadm inside the guest to set up and
manage arrays.

Now mdadm on the host also notices the guest's arrays and wants to auto assemble
them. To prevent this I inserted appropriate ARRAY <ignore> lines into the
host's /etc/mdadm/mdadm.conf.

But I goofed and neglected to tell the host to ignore one of the guest's arrays.
Both the guest and the host ended up assembling the same array simultaneously.
This while I mounted the array read-write in the guest and wrote a bunch of
files to an ext4 fs on it. Only after shutting down the guest did I notice that
the host had also assembled that array. An fsck -f from the host found no 
problems.

So I'm wondering, is it in fact safe to have the same md array assembled in both
the host and the guest simultaneously, as long as only one of them mounts it
read-write?

Thanks,
Frank