On 04/04/2016 02:08 PM, Denis V. Lunev wrote: >> >> This again makes me think this should be a different >> command from something which is obviously useful and >> comprehensible to more than one server/client (i.e. >> allocation). >> > original design of this command has used 16 number > to specify the NUMBER of the bitmap which was > exported by the server. The original design abused the 16-bit 'flags' field of each command to instead try and treat that value as a bitmap number, instead of a bitwise-or'd set of flags. That was one of the complaints against v1, and was fixed in v2 by having a single boolean flag, NBD_CMD_FLAG_DIRTY, which was off for (default) allocation queries, and set for dirtiness queries. We can add other flags for any other type of queries, and the principle of each query being a run-length-encoded listing still applies. > > We have reserved number 0 for 'used' bitmap, i.e. > bitmap of allocated blocks and number 1 for 'dirty' > bitmap. Though we can skip specification of the > belonging of any number except '0' and put them > to server-client negotiations. Or we could reserve > '1' for dirtiness state as server-client agrees and > allow other applications to register their own bitmaps > as the deserve to. > > Why not to do things this original way? If you want to encode particular ids, you should do so in a separate field, and not overload the 'flags' field. As it is, we don't have structured writes - right now, you can write a wire sniffer for the client side, where all commands except NBD_CMD_WRITE are fixed size, and NBD_CMD_WRITE describes its own size via its length field; the extension NBD_CMD_WRITE_ZEROES still fits into this scheme. All NBD implementations have to supply NBD_CMD_WRITE, but any extension commands do NOT have to be universal. Writing a wire sniffer that special-cases NBD_CMD_WRITE is easy (since that command will always exist), but writing a wire sniffer that special-cases arbitrary commands, particularly where those arbitrary commands do not also self-describe the length of the command, is hard. We can't overload the flags field to say which bitmap id to grab, but we also can't arbitrarily add 4 bytes to the command size when the command is NBD_CMD_BLOCK_STATUS (because wire sniffers that don't know about NBD_CMD_BLOCK_STATUS wouldn't know to expect those four bytes to be part of the current packet rather than starting a new packet). The recent work on structured reads made it possible for an arbitrary wire sniffer to gracefully skip over the variable-length return size reply to NBD_CMD_BLOCK_STATUS, and any other extension command that we might add later. But right now, I'm not seeing a compelling reason to add structured commands to the NBD protocol. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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