Why does aufs not write down branches?

nimajnebnima · 09-01-2018, 08:10 PM

I am trying to create a centralized storage pool out of multiple old hard drives I gathered from old computers. To tie them all together, I used aufs to link them into one one mountpoint.

I use Ubuntu 18.04.1.

This is the entries I put in my /etc/fstab for the aufs mount, minus scrubbing the UUIDs, as I use them to mount my drives:

Code:

#mini IDE
UUID= /mnt/ext-2 ext4 defaults 0 2
#COMPTUER-old
UUID= /mnt/COMPUTER-old ext4 defaults 0 2
#old LAPTOP Drive
UUID= /mnt/ext-1 ext4 defaults 0 2
#new LAPTOP Drive - this does not participate in the aufs storage pool
UUID= /mnt/ext-3 ext4 defaults 0 2

#PRESARIO (don't mount)
#UUID= /mnt/PRESARIO noauto 0 0
#UUID= none noauto 0 0

#aufs mount
aufs /mnt/union aufs dirs=/mnt/COMPUTER-old=rw:/mnt/ext-1=rw:/mnt/ext-2=rw,x-systemd.requires-mounts-for=/,x-systemd.requires-mounts-for=/mnt/COMPUTER-old,x-systemd.requires-mounts-for=/mnt/ext-1,x-systemd.requires-mounts-for=/mnt/ext-2,rw,sum 0 0

The mount is sucessful, but anything written to it only go into /mnt/COMPUTER-old. Once that particular mount gets full, aufs errors out with ENOSPC. It's been perplexing me for weeks; why does aufs not write to the rest of the branches I defined, /mnt/ext-1,and /mnt/ext-2? I believe that aufs would write to the next branch I defined in /etc/fstab if the first defined branch failed?

It would be appreciated if one could guide me to the right path to this general idea:

First, write to COMPUTER-old; if unsuccessful, write to ext-1; if unsuccessful, write to ext-2; if unsuccessful, return ENOSPC.

nimajnebnima · 09-02-2018, 03:25 PM

So I just worked around my own problem by adding

Code:

create=mfs

to the aufs mount options in /etc/fstab. It gets the job done of distributing the unified space across all of my hard drives for the time being... but could someone kindly explain why my previous configuration only wrote data to the first drive and not to the other drives when the first drive got full? I expected the branches, from top to bottom, be listed left to right in the /etc/fstab as in the original post; is my expectation correct?