[Qemu-devel] Qcow2 allocate and free cluster

[Gaurab Basu]

Hi,

I have a question regarding how the internal details of the qcow2 image is managed.

My question is how the clusters are allocated. What I have understood, it maintains a free_cluster_index to manage which host clusters are free.
In the function update_refcount(), if a cluster is freed it makes the free_cluster_index point to that cluster. (if cluster_index < free_cluster_index )
For allocating, in the function alloc_clusters_noref(), it uses the free_cluster_index to search for free clusters.

So, suppose I have 1000 clusters which are all allocated, then while freeing, the 2nd cluster gets freed, so the free_cluster_index points to 2nd cluster.
Now, if want to allocate 5 contiguous clusters, i have to search all the way from 2 to 1000 before getting the 5 clusters. 
Am I right in interpreting the mechanism?. If so, isn't it very slow to search for all the clusters everytime.

Thanks and Regards,
Gaurab