On 04/04/2016 03:04 PM, Denis V. Lunev wrote: > In v1 we have had 'status' field, which can have the > following values for dirty request: > > + - `NBD_STATE_DIRTY` (0x0), LBA extent is dirty; > + - `NBD_STATE_CLEAN` (0x1), LBA extent is clean. > > in the extent structure: > > + * 64 bits, offset (unsigned) > + * 32 bits, length (unsigned) > + * 16 bits, status (unsigned)
Between v1 and v2, we dropped 64-bit offset (offset is now implied, by adding lengths of all earlier descriptors), and widened status from 16 bits to 32 bits (so that each descriptor is now naturally 8-byte aligned and therefore easier to make a C array). > > with an additional NBD_STATE_DIRTY_HOLE or (DIRTY_DEALLOCATED) > we could report the entire state using one query. The user could be > able to read entire state which is useful for backup at once. > > Your current proposal is more tricky and it was misunderstood by Alex: > > + * 32 bits, status flags > > and you describe flags as > > + - `NBD_STATE_HOLE` (bit 0); if set, the block represents a hole > + - `NBD_STATE_ZERO` (bit 1), if set, the block contents read as > > + - `NBD_STATE_CLEAN` (bit 2); if set, the block represents a > and opinion of Alex was that all 3 bits could be set in reply to > NBD_CMD_BLOCK_STATUS > with NBD_CMD_FLAG_STATUS_DIRTY set. > > This confused him. This confuses me too. There's nothing that says that NBD_STATE_CLEAN can't be reassigned to bit 0. Conversely, we may want to add a future NBD_CMD_FLAG_FOO that lets you read allocation and dirty information in the same call, in which case having the bits be distinct will make that easier; but where we would also make it obvious that the server is allowed to reject that command flag as unsupported (we already state the server can reject NBD_CMD_FLAG_STATUS_DIRTY with EINVAL as unsupported; and that it if does not reject a dirtiness request but cannot otherwise report anything, then the entire region is reported as dirty). I don't have any strong opinions on whether NBD_STATE_CLEAN should remain bit 2 or be renumbered to bit 0, although renumbering it to bit 0 would make it painfully obvious that you cannot query allocation and dirtiness at the same time. > > If allocated state is not replied to command with NBD_CMD_FLAG_STATUS_DIRTY > then why to have different meaning of bits. Because we still have room - no need to overlap the meaning of bits as long as we have more bits to choose from. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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