On Thu, May 28, 2015 at 4:17 PM, Eneko Lacunza <elacu...@binovo.es> wrote: > On 28/05/15 15:01, Stanislav German-Evtushenko wrote: >>> >>> Note that without O_DIRECT you won't get a "correct" result either; disk >>> may end not containing the data in the buffer when write was called. >>> Softmirror data will be identically uncertain :) >> >> You are right. That is why I suppose there is a bug (operations with >> buffer are not ensured to be thread-safe) in QEMU-KVM. >> > What does it mean that operations with buffer are not ensured to be > thread-safe in qemu? > > O_DIRECT doesn't guarantee that buffer reading is finished when write > returns if I read "man -s 2 open" correctly.
The statement seems to be not correct. O_DIRECT does not guarantee that data reaches the undererying hardware on return but it does guarantee that reading from buffer is finished. Consinder the following examples. This is okay: 0. set_buffer 1. write_with_o_direct_from_buffer 2. change_buffer 3. write_with_o_direct_from_buffer 4. change_buffer 5. write_with_o_direct_from_buffer 6. ... This is not okay and this is what is actually happening: 0. set_buffer 1. start_writing_with_o_direct_from_buffer 2. change_buffer (we can do this only in another thread) 3. finish_writing_with_o_direct_from_buffer 4. change_buffer 5. start_writing_with_o_direct_from_buffer 6. change_buffer (we can do this only in another thread) 7. finish_writing_with_o_direct_from_buffer 8 ... Stanislav _______________________________________________ pve-devel mailing list pve-devel@pve.proxmox.com http://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel
