[sheepdog] vdi object journaling and atomic write

[Liu Yuan]

>From the trace we can see that journaling is most time consuming process for
VDI object write, account almost for 70% (with 3 copies, all 3 nodes on the same box).

Because we support all or nothing atomic write, do we still need to suppose VDI object
journeling?

tailai.ly at taobao:~/sheepdog$ echo (98566+51)/144073 | bc -l
.68449327771338141081

...
gway 27             |            |                     jrnl_begin() {
gway 27             |            |                        xzalloc() {
gway 27             |            |                           xmalloc() {
gway 27             |       1.832|                           }
gway 27             |       4.182|                        }
gway 27             |            |                        xpwrite() {
gway 27             |   26350.316|                        }
gway 27             |            |                        xpwrite() {
gway 27             |   22670.235|                        }
gway 27             |   98566.818|                     }
gway 27             |            |                     default_write() {
gway 27             |            |                        get_obj_path() {
gway 27             |       1.878|                        }
gway 27             |            |                        xpwrite() {
gway 27             |   45129.190|                        }
gway 27             |   45151.567|                     }
gway 27             |            |                     jrnl_end() {
gway 27             |            |                        jrnl_remove() {
gway 27             |      46.592|                        }
gway 27             |      51.4  |                     }
...
gway 27             |  144073.155|   }

Thanks,
Yuan