Disclaimer: This is probably crazy from a performance point of view, also from a reliability point of view, and maybe only of academical value (as in: would it work at all?). That being said:
Will mdadm support a RAID setup with its individual redundant disks being network drives?
Here's the background of my (crazy?) idea: I would like to use one linux box with just a small disk to hold the system. Connected to this box, via ethernet, are two other machines, each with a RAID-0 system to get some use out of old-ish hard drives. Both RAID-0 volumes are built to have approximately the same size. Now, each of these machines offer the RAID-0 volume as a network share, and the box mounts them as /old_drives_0 and /old_drives_1, respectively.
On top of that, I would like to tell mdadm on box to build a RAID-1 system using /old_drives_0 and /old_drives_1 as the two mirrored "disks" and offer the such-created RAID-1 volume on the network as a drive.
Can I tell mdadm to use just about any mount point (and not the usual /dev/sdaX), as in:
mdadm /dev/md0 --create --raid-devices=2 --level=1 /old_drives_1 /old_drives_2
mkfs /dev/md0 # optionally specifying -text3 or -text4
mount /dev/md0 /raid
(loosely quoting https://unix.stackexchange.com/a/48437/27804)
I know there are many reasons not to build a raid over the network, but there may also be reasons in favor of it: Once a disk breaks, all I have to do is hook up a network cable to a different machine… Also, when I put three NICs in box, it can access its own two RAID "drives" over dedicated ethernet connections and use the third for upstream traffic for access to the RAID-1 volume from its clients.
I could even keep around one "box" and one "machine" as a drop-in replacement which would allow quick maintenance once something fails.
Best Answer
RAID always needs a device (at least if you use md). There are two ways. The probably better one is to use a network block device:
https://en.wikipedia.org/wiki/Network_block_device
You can tell md to use such a device for writing only (because it's too slow for reading) with
mdadm ... --write-mostly.The other option is to create a file on a network volume (file system level) and use it for a loop device.