On 04/05/2016 12:17 AM, Eric Blake wrote: > 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. I think it is worth to do to avoid this type of the confusion.
>> 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. > ok ------------------------------------------------------------------------------ _______________________________________________ Nbd-general mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/nbd-general
