Leveraging LVM Caching
I recently implemented LVM caching based largely on Richard WM Jones' post and thought I'd document my own experience here.
I started with an SSD containing /, /home, and swap, two 2 TB HDDs (sdb, and sdc) and an additional SSD (sdd).
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 447.1G 0 disk
├─sda1 8:1 0 500M 0 part /boot
└─sda2 8:2 0 446.7G 0 part
├─fedora-root 253:0 0 50G 0 lvm /
├─fedora-swap 253:1 0 15.7G 0 lvm [SWAP]
└─fedora-home 253:2 0 381G 0 lvm /home
sdb 8:16 0 1.8T 0 disk
sdc 8:32 0 1.8T 0 disk
sdd 8:48 0 119.2G 0 disk
I then used fdisk to create a Linux partition on each HDD and a Linux LVM partition on the SSD. Though, these partition types don't actually matter much for our purposes beyond looking nice.
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 447.1G 0 disk
├─sda1 8:1 0 500M 0 part /boot
└─sda2 8:2 0 446.7G 0 part
├─fedora-root 253:0 0 50G 0 lvm /
├─fedora-swap 253:1 0 15.7G 0 lvm [SWAP]
└─fedora-home 253:2 0 381G 0 lvm /home
sdb 8:16 0 1.8T 0 disk
└─sdb1 8:17 0 1.8T 0 part
sdc 8:32 0 1.8T 0 disk
└─sdc1 8:33 0 1.8T 0 part
sdd 8:48 0 119.2G 0 disk
└─sdd1 8:49 0 119.2G 0 part
I then created a RAID 1 set using the two HDD partitions.
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
# echo 'DEVICE /dev/sdb1 /dev/sdc1' > /etc/mdadm.conf
# mdadm --detail --scan >> /etc/mdadm.conf
# cat /etc/mdadm.conf
DEVICE /dev/sdb1 /dev/sdc1
ARRAY /dev/md0 metadata=1.2 name=bender:0 UUID=2f9fd7a2:cf0d4dd8:579ec7ce:073d0886
Then I created the LVM volumes and an ext4 filesystem on the RAIDed volume.
# pvcreate /dev/md0 /dev/sdd1
Physical volume "/dev/md0" successfully created
Physical volume "/dev/sdd1" successfully created
# vgcreate data /dev/md0
Volume group "data" successfully created
# lvcreate -n vol0 -L 1.5T data /dev/md0
Logical volume "vol0" created.
# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
vol0 data -wi-a----- 1.50t
home fedora -wi-ao---- 380.95g
root fedora -wi-ao---- 50.00g
swap fedora -wi-ao---- 15.69g
# pvs
PV VG Fmt Attr PSize PFree
/dev/md0 data lvm2 a-- 1.82t 326.89g
/dev/sda2 fedora lvm2 a-- 446.64g 4.00m
/dev/sdd1 lvm2 --- 119.24g 119.24g
# mkfs.ext4 /dev/data/vol0
mke2fs 1.42.13 (17-May-2015)
Creating filesystem with 402653184 4k blocks and 100663296 inodes
Filesystem UUID: 51e8796a-5090-45ce-ae62-4944f2eb9132
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
102400000, 214990848
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
Now for the caching configuration. It's important to specify the actual partition (sdd1) in the following commands to ensure that the cache gets created on that device and not arbitrarily within the volume group.
# vgextend data /dev/sdd1
Volume group "data" successfully extended
# lvcreate -L 1G -n vol0_cache_meta data /dev/sdd1
Logical volume "vol0_cache_meta" created.
# lvcreate -L 115G -n vol0_cache data /dev/sdd1
Logical volume "vol0_cache" created.
# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
vol0 data -wi-a----- 1.50t
vol0_cache data -wi-a----- 115.00g
vol0_cache_meta data -wi-a----- 1.00g
home fedora -wi-ao---- 380.95g
root fedora -wi-ao---- 50.00g
swap fedora -wi-ao---- 15.69g
# lvconvert --type cache-pool --poolmetadata data/vol0_cache_meta data/vol0_cache
WARNING: Converting logical volume data/vol0_cache and data/vol0_cache_meta to pool's data and metadata volumes.
THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
Do you really want to convert data/vol0_cache and data/vol0_cache_meta? [y/n]: y
Converted data/vol0_cache to cache pool.
# lvconvert --type cache --cachepool data/vol0_cache data/vol0
Logical volume data/vol0 is now cached.
# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
vol0 data Cwi-a-C--- 1.50t [vol0_cache] [vol0_corig] 0.00 1.43 100.00
home fedora -wi-ao---- 380.95g
root fedora -wi-ao---- 50.00g
swap fedora -wi-ao---- 15.69g
Then I mounted it at /var/data.
# echo '/dev/mapper/data-vol0 /var/data ext4 defaults 1 2' >> /etc/fstab
# mount -a
# df -h /var/data
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/data-vol0 1.5T 69M 1.5T 1% /var/data