On 02/18/2016 12:18 PM, Roman Kagan wrote: > On Wed, Feb 17, 2016 at 01:58:47PM -0700, Eric Blake wrote: >> On 02/17/2016 11:10 AM, Denis V. Lunev wrote: >>> @@ -446,6 +448,11 @@ The following request types exist: >>> about the contents of the export affected by this command, until >>> overwriting it again with `NBD_CMD_WRITE`. >>> >>> +* `NBD_CMD_WRITE_ZEROES` (6) >>> + >>> + A request to write zeroes. The command is functional equivalent of >>> + the NBD_WRITE_COMMAND but without payload sent through the channel. >> This lets us push holes during writes. Do we have the converse >> operation, that is, an easy way to query if a block of data will read as >> all zeroes, and therefore the client can bypass reading that portion of >> the disk (in other words, an equivalent to lseek(SEEK_HOLE/SEEK_DATA))? > The spec doesn't have anything like that. > > OTOH, unlike the write case, where you have all the information and just > choose whether to send normal write or zero write, the extra round-trip > of a separate SEEK_HOLE/SEEK_DATA request may lead to actually degrading > the overall throughput. > > Rather it may be a better idea to add something like sparse read where > the server would, instead of sending the full length of data in the > response payload, send a smarter variable-length package with a > scatter-gather list or a bitmap of used blocks in the beginning, and let > the client decode it and fill the gaps with zeros. > > Roman. ah, I see.
This story is more difficult but also viable for backup dirty bitmap reading. But this will make the protocol more complex and will require more efforts at specification stage. I'd better start with the current change, which is simple enough and make changes in a right direction and after that continue with READ2 or whatever command. Den ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=272487151&iu=/4140 _______________________________________________ Nbd-general mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/nbd-general
