On Fri, 03/17 12:20, Peter Lieven wrote: > Am 17.03.2017 um 12:16 schrieb Paolo Bonzini: > > > > On 17/03/2017 12:11, Peter Lieven wrote: > >>>> like VMDK or QCOW2 shouldn't we trust the information from the l2 tables > >>>> in the VMDK or QCOW2? > >>> It provides additional information, for example it works better with > >>> prealloc=metadata. > >> Okay, understood. Can you imagine of a away to conditionally avoid this > >> second callout? In my case we have an additional > >> lseek for each cluster. For a 20GB file this are approx. 327k calls to > >> lseek. And if the file has no preallocated metadata > >> it will likely not improve anything. And even if the metadata is > >> prealloced what is the allocation status of the clusters? > > If the metadata is preallocated, cluster will (or should) show up as > > zero, speeding up the copy. > > Okay, in this case the second call out to *file will not happen. It only > happens if the metadata says it contains data. > So where does it actually help? > > The condition is: (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) && (ret > & BDRV_BLOCK_OFFSET_VALID)) > > So from my view it can only have any effect if the metadata returns > BDRV_BLOCK_DATA, but the protocol driver returns > BDRV_BLOCK_ZERO. > > This can only happen if I partially write to a cluster, or am I wrong here?
I think you have a point. The metadata should have said BDRV_BLOCK_ZERO if protocol would say BDRV_BLOCK_ZERO - there is no reason the format driver cannot know. Fam
