cache=none or cache=Write Back.

[fusion]

Hello ,

I have installed Proxmox with ZFS RAID Z1.

I have a doubt.

In a KVM Virtio, always use VirtIO = Write Back.

But doing tests, I see Default(no cache). It is much faster write speed.

test: dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync

cache: none = 1073741824 bytes (1.1 GB) copied, 0.813185 s, 1.3 GB/s
cache:Write Back = 1073741824 bytes (1.1 GB) copied, 2.37712 s, 452 MB/s

It is normal? I have to use cache: none? or better another config?

Thank you.

 

[LnxBil]

a) don't test with dd and b) don't use /dev/zero to test on ZFS. Zero is not written, but stripped away.

 

[mir]

It is normal? I have to use cache: none? or better another config?

I have made extensive tests for cache setting and ZFS. cache: none always gave best performance so I use cache: none as default cache setting.

 

[fusion]

if I also noted that with cache: none has more performance.

how do I a speed test zfs disk?

 

[LnxBil]

With e.g. fio. @mir posted a working configuration on various occasions. You'll use it with a test file on a ZFS filesystem or directly on a zvol.

 

[gkovacs]

I have made extensive tests for cache setting and ZFS. cache: none always gave best performance so I use cache: none as default cache setting.

Could you share some numbers from your extensive testing?

Because the test results I saw point to cache=writeback being the winner, even on ZFS:
http://jrs-s.net/2013/05/17/kvm-io-benchmarking/

 

[mir]

Could you share some numbers from your extensive testing?

Because the test results I saw point to cache=writeback being the winner, even on ZFS:
http://jrs-s.net/2013/05/17/kvm-io-benchmarking/

First, the benchmark you refer to is 3 years old.
Second, all tests is done exclusively using Windows guests.
Third, the benchmarker is referring to cache = none and cache = nocache as 2 separate things. This is wrong, cache = none and cache = nocache is the same.
Forth, He clams ZFS does not support DirectIO which is completely wrong.

My numbers was made some while ago so I cannot produce the numbers here since I did not save them. Apart from that my setup is exclusively ZFS over iSCSI which might behave different from ZFS as local filesystem.