Re: [Qemu-devel] [PATCH v3 2/6] add basic backup support to block driver

[Dietmar Maurer]

> > I just posted v5 of the patch. But I get a slow down of 15% if I use
> > bdrv_is_allocated_above. (tested with empty qcow2 files.)
> 
> Strange, for an unallocated area bdrv_is_allocated_above and bdrv_read really
> do the same thing apart from writing the zeroes to the buffer.
> The code looks okay, does a profile say where the time is being spent?
> Or does the is_allocated_above ever trigger (are you using metadata
> preallocation)?

Yes, I use metadata preallocation. But that does not explain why it gets slower?

> BTW, any reason why BACKUP_BLOCKS_PER_CLUSTER is hardcoded and you're
> not using the cluster size from bdrv_get_info?

A backup includes several block devices, and I want to have the same cluster 
size
for all. It makes the code easier.