KVM performance “qcow2″ vs “raw”, “ide” vs “virtio”
March 20th, 2010
I just got a new server for private use and needed to put some virtual machines, I decided to go with KVM as it seems it will be supported longer than XEN
Of course regardless of virtualization platform the decision has to be made – how to store Virtual Disks?
KVM wiki suggests that qcow2 is fast enough, is it really?
Some benchmarks of clean install install of Ubuntu 9.10 with KVM, Virtual Disk stored in qcow2 format:
qcow2+ide
Version 1.03c      ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine       Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
entropy.be      4G 38684 43 53081  9 64187 13 55412 89 360629 43 5278 12
qcow2+virtio
Version 1.03c      ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine       Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
entropy.be      4G 58859 73 76112 13 74583 14 52058 91 546065 44 5725 32
raw(file)+ide
Version 1.03c ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
entropy.be 4G 74111 83 86947 15 66292 14 50945 89 347763 41 5892 7
raw(file)+virtio:
Version 1.03c ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
entropy.be 4G 74607 90 86478 16 20930 7 51069 88 658254 48 8676 64
as a comparison, native I/O on host filesystem:
Version 1.03c ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
serwer3.itcoms. 16G 75336 82 89196 14 34498 5 46891 79 99439 5 421.1 1
It seems qcow2 perfomance in block input and output is still slightly worse than raw image. Also virtio is faster than ide for qcow2.
All benchmarks were run using `bonnie++ -u root -d /home` using kernel 2.6.31-20-server #58-Ubuntu SMP Fri Mar 12 05:40:05 UTC 2010 x86_64 GNU/Linux
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