Dual Disk LVM with Loopback Files
The story
My SPARC Enterprise T5120 came with two 10000 RPM SAS HDDs, both with a labeled capacity of 146GB. Gentoo was installed on the first
hard drive, and thanks to the old SILO that was in charge of booting the system up, I have to keep a single-large-root partition
scheme. While the second disk seemed to be in some odd state (it had a block size of 528) and can't get recognized by Linux's SCSI
driver, I got it formatted with sg_format, turning it into a data disk with single partition on disk with ext4, and used it as the
default download location for Deluge, my BitTorrent client. So far, so good.
Yet 146GB is damn too tight these days, and soon it got filled up. Yet, the first disk, which was completely used as the root drive, had around 130GB to spare. So is there a plan so that I can utilize all the free space on the two disks, say, to combine them into a big new pool?
The solution
Fortunately, LVM comes to the rescue. It's trivial to create empty files, set them up as loopback devices, and make them considered pv's
by the system. Thanks to truncate, we can create files with big sizes, yet doesn't take up actual space.
Creating the filesystem
cd /
truncate pool_member.img -s 135G # won't consume disk space
losetup /dev/loop0 /pool_member.img
cd /var/lib/deluge # mountpoint for /dev/sdb1
truncate pool_member.img -s 135G # won't consume disk space
losetup /dev/loop1 /var/lib/deluge/pool_member.img
pvcreate /dev/loop0
pvcreate /dev/loop1
vgcreate data /dev/loop0 /dev/loop1
lvcreate -i 2 -l +100%FREE -n download data
mkfs.ext4 /dev/data/download
mkdir -p /pool
mount /dev/data/download /pool
Check with df -h and we can see that there's a brand new filesystem mounted there, taking space from the two drives. What I had to do
next was simply moving the existing torrents over with Deluge's "Move Storage" feature.
Automatically mounting the LV
Wait a second. Even though the volume is up and running, it would be too much labor having to do it manually every time the system reboots. The following init script for OpenRC works well to solve this problem:
#!/sbin/openrc-run
# Copyright 1999-2018 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
lv_file="/dev/data/download"
wait_file() {
local file="$1"; shift
local wait_seconds="${1:-10}"; shift # 10 seconds as default timeout
until test $((wait_seconds--)) -eq 0 -o -e "$file" ; do sleep 1; done
((++wait_seconds))
}
depend() {
need localmount
before net
}
start() {
ebegin "Starting mount local disk pool"
losetup /dev/loop0 /pool_member.img
losetup /dev/loop1 /var/lib/deluge/pool_member.img
vgscan
einfo "Waiting for LV to appear..."
wait_file "$lv_file" 5 || {
eend 1 "LV did not show up after waiting for 5 seconds"
}
mount /dev/data/download /pool
eend $? "Failed to mount /pool"
}
stop() {
ebegin "Stopping mount local disk pool"
umount /pool
vgchange -an /dev/data
losetup -d /dev/loop0
losetup -d /dev/loop1
eend $? "Failed to unmount /pool"
}
Save it to /etc/init.d/disk-pool, give it +x permission, and add it to the default runlevel with
rc-update add disk-pool default
Credit
Credit for this method goes to lilydjwg. Thanks!