On 15 Apr 2016, at 15:10, Eric Blake <ebl...@redhat.com> wrote: > You completely ignore NBD_CMD_FLAG_NO_HOLE; you could obey that flag > (when set, do the calloc() here; when cleared, call into the trim code > if you can guarantee that you'll read zero after trims).
Sadly you cannot guarantee that you will read zeroes after exptrim is called. Exceptions include: 1. F_COPYONWRITE being set (trim is ignored) 2. The initial and final blocks if F_TREEFILES is set if either of the start and end of the read is not aligned to TREEPAGESIZE 3. Any underlying filesystems which don't support fallocate with PUNCH_HOLE (we don't check the error return) 4. Probably something on Windows (!) > However, the client MAY set the command flag NBD_CMD_FLAG_NO_HOLEto inform > the server that the area MUST be fully provisioned, ensuring that future > writes to the same area will not cause fragmentation or cause failure due to > insufficient space. So I could do this: > But if nothing > else, you have a bug for not permitting the NBD_CMD_FLAG_NO_HOLE (even > if you intend to ignore it by ALWAYS operating in no-hole mode, as was > done here). Or I could do the physical write of zeroes and then call trim afterwards if NBD_CMD_FLAG_NO_HOLE is *not* set and trim is supported. As fallocate() and friends are cheap, I think I will do that. Someone should really do this properly (which is non-trivial); I was really getting a baseline code example for the extension branch demonstration. -- Alex Bligh
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