On Mon 30 Jan 2017 05:14:41 PM CET, Alberto Garcia wrote: > This patch keeps the index of the last used (i.e. non-zero) entry in > the refcount table and updates it every time the table changes. The > refcount-block overlap check then uses that index instead of reading > the whole table.
Note that while I decided to go for this approach the patch can be made much simpler by simply stopping at the first empty entry in the refcount table: if ((chk & QCOW2_OL_REFCOUNT_BLOCK) && s->refcount_table) { for (i = 0; i < s->refcount_table_size; i++) { if (!(s->refcount_table[i] & REFT_OFFSET_MASK)) { break; } if (overlaps_with(s->refcount_table[i] & REFT_OFFSET_MASK, s->cluster_size)) { return QCOW2_OL_REFCOUNT_BLOCK; } } } I don't think QEMU produces files where refcount_table[i] == 0 but refcount_table[i + 1] != 0. Do they even make sense? In any case, my patch would cover those cases too, but this simplified version wouldn't. Berto