On Mon, 03/20 12:21, Paolo Bonzini wrote: > > > On 20/03/2017 03:46, Fam Zheng wrote: > > 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. > > That's true of qcow2, but many formats (including raw :)) don't know > about BDRV_BLOCK_ZERO.
Raw is a little special, it could have forwarded the call to *file in its BlockDriver callback. Most formats with metadata stores zero/nonzero information in L1/L2 tables. For qcow2 and VMDK I think it's okay to just trust meta data on zero/nonzero. Fam
