[libvirt-users] preallocation=full Vs preallocation=metadata

[Kashyap Chamarthy]

On Tue, Jun 02, 2015 at 09:14:03AM +0530, Jatin Davey wrote:
> Hi All
> 
> I was reading through this:
> 
> http://kashyapc.com/2011/12/02/little-more-disk-io-perf-improvement-with-fallocateing-a-qcow2-disk/

In my testing, the above method gives near-raw performance as it
preallocated all the space ahead of time.

The above test can now be done in a single command -- see option (3)
below.

> I was basically searching for pointers on improving disk I/O.
> 
> I wanted to know the purpose of preallocation=full & preallocation=metadata >
>
> , What is the difference between them ? and which one would yield a better
> disk I/O speed ?

There are three options that modern `qemu-img` supports:

(1) 'preallocation=metadata': allocates qcow2 metadata, and it's still
    a sparse image.

    $ qemu-img create -f qcow2 -o preallocation=metadata test1-metadata.qcow2 1G
    Formatting 'test1-metadata.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 preallocation='metadata' lazy_refcounts=off refcount_bits=16
    328K -rw-r--r--. 1 root root 1.1G Jun  3 03:20 copy-test1-metadata.qcow2

(2) 'preallocation=full': allocates zeroes and makes a non-sparse image.

    $ qemu-img create -f qcow2 -o preallocation=full test2-full.qcow2 1G
    Formatting 'test2-full.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 preallocation='full' lazy_refcounts=off refcount_bits=16
    $ ls -lash test2-full.qcow2 
    1.1G -rw-r--r--. 1 root root 1.1G Jun  3 03:31 test2-full.qcow2

(3) 'preallocation=falloc': which uses posix_fallocate() to "allocate
    blocks and marking them as uninitialized", and is relatively faster
    than writing out zeroes to a file:

    $ qemu-img create -f qcow2 -o preallocation=falloc test3-falloc.qcow2 1G
    Formatting 'test3-falloc.qcow2', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 preallocation='falloc' lazy_refcounts=off refcount_bits=16
    $ ls -lash test3-falloc.qcow2 
    1.1G -rw-r--r--. 1 root root 1.1G Jun  3 03:32 test3-falloc.qcow2

You can test and compare between (2) and (3) what works best for you.

-- 
/kashyap