LVM CHEATSHEET

LVM is the "Logical Volume Manager". Its a suite of tools that make for easy and powerful management of disks and pools of data.

GLOSSARY

ACRONYM	NAME	DESCRIPTION
pv	physical volume	The physical disk or partition in array
vg	volume group	The name of the whole array
lv	logical volume	The name of a virtual disk within the array. More than one LV may be in a VG

CREATING A NEW LV

Create PV(s)
```
sudo pvcreate /dev/sdX
```
Create VG - give it the disk names for the PVs at hand, one or more PVs can be added initially
```
sudo vgcreate VG-NAME /dev/sdX /dev/sdY
```
Add PV(s) to VG
```
sudo vgextend VG-NAME /dev/sdZ
```

Create LV-NAME

#Non-mirrored LV - (100MB in example, and %100 afterwards) in VG-NAME
sudo lvcreate -L 100M -n LV-NAME VG-NAME
sudo lvcreate -l 100%FREE -n LV-NAME VG-NAME

#Create a single mirror lv.
#-- You need .1% roughly for log file if keeping on same set.
#-- Presuming 100G VG.
#-- You can add "--nosync" to the command to skip initial sync.
sudo lvcreate -L 49G -m1 -n LV-NAME VG-NAME

Format lv as ext4 or xfs

sudo mkfs.ext4 /dev/VG-NAME/LV-NAME && e2label /dev/VG-NAME/LV-NAME labelname
sudo mkfs.xfs /dev/VG-NAME/LV-NAME

MIRROR EXISTING LV

Reduce filesystem

#Use 'df' to see where things are mounted
sudo umount LV-MOUNTPOINT

#Resize filesystem on LV
sudo fsadm resize /dev/VG-NAME/LV-NAME NEWSIZE

#OPTIONAL :
#Resize both LV and filesystem to 5G (POTENTIALLY UNSAFE)
#-- Skip to step 4 afterwards
sudo lvreduce --resizefs -L 5G /dev/VG-NAME/LV-NAME

Reduce LV

#Reduce LV by 5G (Sufficient for ~ 5TB @ .1% rule)
sudo lvreduce -L -5G /dev/VG-NAME/LV-NAME

#Reduce LV to 5G
sudo lvreduce -L 5G /dev/VG-NAME/LV-NAME

Resize filesystem
```
sudo fsadm resize /dev/VG-NAME/LV-NAME
```
Create pv for new drive(s). One more drive may be necessary if you want to create a mirrorlog disk.
```
sudo pvcreate /dev/sdX
```
Add new pv(s) to vg
```
sudo vgextend /dev/VG-NAME /dev/sdX
```

Create mirror using new drive(s)

sudo lvconvert -m1 /dev/VG-NAME/LV-NAME

MIGRATION FROM OLD DISKS IN VG TO NEW ONES

Create PV for new disk
```
sudo pvcreate /dev/sdz
```
Extend the VG to include the new disk
```
sudo vgextend VG-NAME /dev/sdz
```
Migrate extents off old disk
```
sudo pvmove /dev/sdb
```
Remove PV from LV
```
sudo vgreduce VG-NAME /dev/sdb
```
Remove PV from VG & validate
```
sudo pvremove /dev/sdb
sudo pvdisplay
```

MIGRATION OF LVM ARRAY TO NEW HOST

Unmount the LVM VG(s) in question, and validate that it is indeed unmounted
```
sudo umount /dev/VG-NAME/LV-NAME
df -h
```
Mark VG as inactive and verify inactive status
```
sudo vgchange -an VG-NAME
sudo lvdisplay
```
Export VG and verify state
```
sudo vgexport VG-NAME
sudo vgs
```
Power off hardware and transfer it to new host
Import VG and verify import status
```
sudo vgimport VG-NAME
sudo vgs
```
Mark VG as active and verify active status
```
sudo vgchange -ay VG-NAME
sudo vgs
```

Mount VG and verify mount status

sudo mkdir -p /mnt/VG-NAME
mount /dev/VG-NAME/LV-NAME /mnt/VG-NAME/LV-NAME
df -h

LV CACHE

PREFACE

To add a cache to an existing VG we will perform a few steps to create the cache and metadata LVs and correctly tag these two LVs as cache mode. This guide will presume this is a fresh drive not in any existing VG. If it is, remove it first, then the drive can be utilized.

CREATING THE CACHE LVs

Remove disk from any other VGs. Use 'pvs -a' to show your PVs and their allocations
```
vgreduce OLD_VG /dev/sdX
```
Create pv
```
pvcreate /dev/sdX
```
Extend vg
```
vgextend VG_NAME /dev/sdX
```
Create lv for cache metadata in lv. For YMB, use a value for Y that is .1% of total cache (1GB for a 1TB disk, etc)
```
lvcreate -n cache_meta -L YMB VG_NAME /dev/sdX
```

Create a cache data LV

lvcreate -n cache -l 99%PVS VG_NAME /dev/sdX

Create the cache pool LV

lvconvert --type cache-pool --poolmetadata VG_NAME/cache_meta VG_NAME/cache

Create cache

lvconvert --type cache --cachepool VG_NAME/cache VG_NAME/CACHE_TARGET

Verify

        #lvs
        LV      VG      Attr       LSize  Pool    Origin          Data%  Meta%  Move Log Cpy%Sync Convert
        LV_NAME VG_NAME Cwi-aoC--- <3.42t [cache] [LV_NAME_corig] 0.02   0.62            0.00

BREAK LVM CACHE

To remove the LVM cache is straighforward:

lvconvert --uncache VG_NAME/LV_NAME
vgreduce VG_NAME /dev/sdX

Done!

QUICK TASKS HOWTO

Add a disk to the VG and extend the LV out to make use of it

sudo pvcreate /dev/sdX
sudo vgextend VG_NAME /dev/sdX
sudo lvextend -l +100%FREE VG_NAME/LV_NAME
sudo fsadm resize /dev/VG-NAME/LV-NAME

Remove disk from VG. Note, you will need to have the available space to hold all LV's in the VG after disk removal.
```
sudo pvmove /dev/sdX
sudo vgreduce VG_NAME /dev/sdX
sudo pvremove /dev/sdX
```
Swap smaller disk(s) for larger one(s)
1. pvcreate on each new disk.
2. vgextend each new disk
3. pvmove each old disk that you wish to remove
4. lvextend if need be
5. extend filesystem if need be

REFERENCES

https://www.redhat.com/en/blog/improving-read-performance-dm-cache

TROUBLESHOOTING

CANNOT CREATE PV

If you get the message:

    $ sudo pvcreate /dev/sdb /dev/sdc
      Device /dev/sdb not found (or ignored by filtering).
      Device /dev/sdc not found (or ignored by filtering).

The solution is to recreate the disklabel from gpt to msdos:

    $ sudo parted /dev/sdb
    GNU Parted 3.1
    Using /dev/sdb
    Welcome to GNU Parted! Type 'help' to view a list of commands.
    (parted) mklabel msdos
    Warning: The existing disk label on /dev/sdb will be destroyed and all data on this disk will be lost. Do you want to continue?
    Yes/No? y                                                                 
    (parted) select /dev/sdc                                                  
    Using /dev/sdc
    (parted) mklabel msdos
    Warning: The existing disk label on /dev/sdc will be destroyed and all data on this disk will be lost. Do you want to continue?
    Yes/No? y                                                                 
    (parted) q                                                                
    Information: You may need to update /etc/fstab.

You likely will need a reboot, but then the disks should be able to be turned into a PV without issue:

    $ sudo pvcreate /dev/sdb
    WARNING: dos signature detected on /dev/sdb at offset 510. Wipe it? [y/n]: y
      Wiping dos signature on /dev/sdb.
      Physical volume "/dev/sdb" successfully created.
    $ sudo pvcreate /dev/sdg
    WARNING: dos signature detected on /dev/sdg at offset 510. Wipe it? [y/n]: y
      Wiping dos signature on /dev/sdg.
      Physical volume "/dev/sdg" successfully created.

All fixed!