#thin pool

Enabling fstrim on various storage layers

dm-crypt

/etc/crypttab sda1_crypt         /dev/sda1          none             luks,discard

LVM

/etc/lvm/lvm.conf (...) devices {   (...)   issue_discards = 1   (...) } (...)

Filesystem

Fstrim on any filesystem can be enabled by adding discard flag to fstab. Since it will trigger defragmentation and deletion of freed blocks on every remove operation at filesystem level, it’s highly discouraged to do so. Instead one should run fstrim command periodically. Shedule should be adjusted to a particular use case. Command to check which devices support fstrim:

#lsblk -o KNAME,DISC-MAX KNAME DISC-MAX sda         2G sda1        2G sda2        2G dm-0        2G dm-1        2G

#cat /sys/block/sda/queue/discard_max_bytes 2147450880

Those with DISC-MAX of non zero value do support fstrim.

QEMU

To leverage fstrim inside domain we have to setup disk bus to utilise SCSI and SCSI controller to VirtIO SCSI:

<controller type='scsi' index='0' model='virtio-scsi'>   <alias name='scsi0'/>    <address type='pci' domain='0x0000' bus='0x02' slot='0x03' function='0x0'/> </controller> <disk type='network' device='disk'> (..)   <target dev='sda' bus='scsi'/> (...)

Then we add discard='unmap' option to disk definition:

<driver name='qemu' type='raw' cache='writeback' discard='unmap'/>

Creating LVM thin pool and thin pool volumes

At first, install thin_check tool, eg. in Ubuntu run:

apt install thin-provisioning-tools

Create optimally aligned partition for physical volume using parted:

# parted -a optimal /dev/sdb mkpart pri 0% 200G

# parted /dev/sdb align-check optimal 1 1 aligned

Create LVM physical volume and volume group:

# pvcreate /dev/sdb1  Physical volume "/dev/sdb1" successfully created.

# vgcreate vg1 /dev/sdb1  Volume group "vg1" successfully created

Create thin pool volume for thin volumes allocation:

# lvcreate --type thin-pool --extents 47635 --name  vg1-thin-pool vg1  Using default stripesize 64,00 KiB.  Thin pool volume with chunk size 128,00 KiB can address at most 31,62 TiB of data.  Logical volume "vg1-thin-pool" created.

Finally create logical volume withing previously created thin pool: