search for: autoclear

...ead as zero. The combination NBD_INIT_SPARSE|NBD_INIT_ZERO is common (generally, if you use lseek(SEEK_DATA) to prove the entire image reads as zeroes, you also know the entire image is sparse), but NBD_INIT_ZERO in isolation is also possible (especially with the qcow2 proposal of a persistent autoclear bit, where even with a fully preallocated qcow2 image you still know it reads as zeroes but there are no holes). But you are also right that for servers that can advertise both bits efficiently, NBD_INIT_SPARSE in isolation may be more common than NBD_INIT_SPARSE|NBD_INIT_ZERO (the former for...

...e combination NBD_INIT_SPARSE|NBD_INIT_ZERO is common (generally, > if you use lseek(SEEK_DATA) to prove the entire image reads as > zeroes, you also know the entire image is sparse), but NBD_INIT_ZERO > in isolation is also possible (especially with the qcow2 proposal of > a persistent autoclear bit, where even with a fully preallocated > qcow2 image you still know it reads as zeroes but there are no > holes). But you are also right that for servers that can advertise > both bits efficiently, NBD_INIT_SPARSE in isolation may be more > common than NBD_INIT_SPARSE|NBD_INIT_ZERO...

...that infrastructure already exists: does block_status at offset 0 and the entire image as length return status that the entire file is a hole. And for qcow2 files, it would not be that hard to teach a similar block_status request to report the entire image as a hole based on my proposed qcow2 autoclear bit tracking that the image still reads as zero. > > At the same time, I still haven’t understood what we need the flag for. > > As far as I understood in our discussion on your qemu series, there is > no case where anyone would need to know whether an image is zero. All > pra...

I will be following up to this email with four separate threads each addressed to the appropriate single list, with proposed changes to: - the NBD protocol - qemu: both server and client - libnbd: client - nbdkit: server The feature in question adds a new optional NBD_INFO_ packet to the NBD_OPT_GO portion of handshake, adding up to 16 bits of information that the server can advertise to the