in my opinion the terms 

raw, qcow2, thin provisioned, sparse, preallocated

are used in sub optimal way in oVirt and RHEV documentation and create confusion for the new incoming user (but not only to him/her)

My referrals for them are:

Sometimes I have to come back to docs to clear my doubts and I'm never fully satisfied.

I report below only some excerpts to explain my findings

oVirt

1) Virtual disks can have one of two formats, either Qcow2 or RAW. The type of storage can be either Sparse or Preallocated. Snapshots are always sparse but can be taken for disks created either as RAW or sparse.

--> From above one concludes that "format" is one thing and "type" is another thing and that all the mix format/type are in general possible (depending on storage layout... see below): Qcow2/Sparse, RAW/Prellocated, Qcow2/Preallocated, RAW/Sparse

But then, both in Ovirt and RHEV page

2) A virtual disk with a preallocated (RAW) format has significantly faster write speeds than a virtual disk with a thin provisioning (Qcow2) format. Thin provisioning takes significantly less time to create a virtual disk. The thin provision format is suitable for non-IO intensive virtual machines.

--> now one concludes instead that format==type and that so

preallocated==RAW

thin provisioning==Qcow2

and also indirectly that thin provisioning == sparse because in point 1) the term sparse was the one used..

Indeed later, in section 12.2 of RHEV and in section "Understanding Virtual Disks" of oVirt guide there is the classification of the 2 "types" again, Preallocated and Thin Provisioned and the latter contains:

"For sparse virtual disks backing storage is not reserved and is allocated as needed during runtime. This allows for storage overcommitment ..."

And this confirms that sparse==thin provisioned

In the same part for both docs

3) Table 11.1. Permitted Storage Combinations for oVirt and

Table 12.1. Permitted Storage Combinations for RHEV

Again there are almost-all the combinations described:

For file based storage (NFS) only Qcow2/Preallocated is not contemplated

For iSCSI/SAN only RAW/Sparse is not contemplated

4) Gluster

last but not least, It is not clear in my opinion where should be put Gluster (file based or SAN based)... I think that depending on POSIXFS_DOMAIN (pre 3.5, correct?) or GLUSTERFS_DOMAIN usage it could match both of them....

As far as format, it seems both qcow2 and raw are possible 

This message is just to share my uncertainty when I have to deal with disks in oVirt and explain/compare with VMware, to make it possible to simplify life to new users.

Coming from VMware world they know about:

- thin provision

- thick provision (lazy zeroed)

- thick provision (eager zeroed)

see also:

In my opinion it could be useful to have a preface with a short description of all the terminology that will be used 

HIH improving docs,

Gianluca