Re: backup_calculate_cluster_size does not consider source

[Max Reitz]

On 06.11.19 11:18, Dietmar Maurer wrote:
>> The thing is, it just seems unnecessary to me to take the source cluster
>> size into account in general.  It seems weird that a medium only allows
>> 4 MB reads, because, well, guests aren’t going to take that into account.
> 
> Maybe it is strange, but it is quite obvious that there is an optimal cluster
> size for each storage type (4M in case of ceph)...

Sure, but usually one can always read sub-cluster ranges; at least, if
the cluster size is larger than 4 kB.  (For example, it’s perfectly fine
to read any bit of data from a qcow2 file with whatever cluster size it
has.  The same applies to filesystems.  The only limitation is what the
storage itself allows (with O_DIRECT), but that alignment is generally
not greater than 4 kB.)

As I said, I wonder how that even works when you attach such a volume to
a VM and let the guest read from it.  Surely it won’t issue just 4 MB
requests, so the network overhead must be tremendous?

Max

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