Re: [Libguestfs] [RFC libnbd PATCH] info: Add support for new qemu:joint-allocation

On Thu, Jun 10, 2021 at 01:20:13AM +0300, Nir Soffer wrote: > > + else if (strcmp (metacontext, "qemu:joint-allocation") == 0) { > > + /* Combo of base:allocation and stripped-down qemu:allocation-depth */ > > + const char *base, *depth; > > + switch (type & 3) { > > + case 0: base = "allocated"; break; > > + case 1: base = "hole"; break; > > + case 2: base = "zero"; break; > > + case 3: base = "hole,zero"; break; > > + } > > + switch (type & 0xc) { > > + case 0: depth = "unallocated"; break; > > Is this possible? qemu reports BDRV_BLOCK_DATA but not BDRV_BLOCK_ALLOCATED? No, qemu should never report a status of 0 (which in this code would produce the string "allocated,unallocated", although a v2 may change to print "<unexpected value>"). Remember, BDRV_BLOCK_ALLOCATED is a bit of a misnomer - it has nothing to do with whether a cluster occupies allocated space, but rather whether the local image in the backing chain provides the contents of the cluster (rather than deferring to the backing chain). The code in block/io.c guarantees that if a block device reports BDRV_BLOCK_DATA, then the block layer also reports BDRV_BLOCK_ALLOCATED (that is, any cluster that provides guest-visible data by necessity implies that the current layer of the backing chain is important). However, it DOES point out that "allocated" might not be the best name in libnbd; perhaps "data" or "normal" would be better for the NBD base:allocation status of 0.