For my next linux servers I’m exploring zfs as storage option. There is some stuff on zfs which you need to decide on zpool creation. This can’t be changed later without destroying your data.
The block size of the filesystem is one of these. In zfs this is named
With this property you can balance the space which very small files
take and the MB/s you get when reading very big files. The price you
pay for that is disk space.
When you google around
ashift=12 is recommamed today for hard disks,
mainly because big hard disks have nowadays 4096 bytes sectors (ashift=12, 2^12) as default.
But I also often read that
ashift=13 is recommended for SSD’s.
I use zfs on SSD’s and normal hard drives. So
ashift=13 for SSD’s?
Here some examples how the different
ashift values influence your disk space.
- ashift 9, 512 bytes block size
- ashift 12, 2048 bytes block size
- ashift 13, 4096 bytes block size
Go here if you want to see this post in german.
linux source code
Here I use the linux source code as testing data. I use three copies of the source code.
Here as plain numbers.
tar xf /tmp/linux-4.0.tar.xz cp -Ra linux-4.0/ linux-4.0.1 cp -Ra linux-4.0/ linux-4.0.2 # ashift 9 $ zfs list NAME USED AVAIL REFER MOUNTPOINT tank 742M 324G 741M /tank # ashift 12 $ zfs list NAME USED AVAIL REFER MOUNTPOINT tank 1.08G 324G 1.08G /tank # ashirt 13 $ zfs list NAME USED AVAIL REFER MOUNTPOINT tank 1.54G 323G 1.54G /tank
With a lot of small file the space savings you get with
ashift=9 are really big.
You need twice as much space with
ashift=13 instead of
a raw image file
Here I use a qemu raw image file.
Here as numbers.
# ashift 9 $ zfs list NAME USED AVAIL REFER MOUNTPOINT tank 2.82G 322G 2.82G /tank # ashift 12 zfs list NAME USED AVAIL REFER MOUNTPOINT tank 2.88G 322G 2.88G /tank # ashift 13 zfs list NAME USED AVAIL REFER MOUNTPOINT tank 2.94G 322G 2.94G /tank
sample production data
This testing data consists of some django project data files, some typo3 project files and linux binaries.
# ashift 9 root@nara /tank # zfs list NAME USED AVAIL REFER MOUNTPOINT tank 5.28G 320G 5.28G /tank # ashift 12 zfs list NAME USED AVAIL REFER MOUNTPOINT tank 5.67G 319G 5.67G /tank # ashift 13 zfs list NAME USED AVAIL REFER MOUNTPOINT tank 6.13G 319G 6.13G /tank
I will use
ashift=12 for SSD’s. For the SSD’s capacity I use, the space
price to pay for
ashift=13 is too high for me.