[Gluster-devel] Gluster AFR volume write performance has been seriously affected by GLUSTERFS_WRITE_IS_APPEND in afr_writev

[li.ping288 at zte.com.cn]

GLUSTERFS_WRITE_IS_APPEND Setting in afr_writev function at glusterfs 
client end makes the posix_writev in the server end  deal IO write fops 
from parallel  to serial in consequence.

i.e.  multiple io-worker threads carrying out IO write fops are blocked in 
posix_writev to execute final write fop pwrite/pwritev in __posix_writev 
function ONE AFTER ANOTHER. 

For example:

thread1: iot_worker -> ...  -> posix_writev()   |
thread2: iot_worker -> ...  -> posix_writev()   |
thread3: iot_worker -> ...  -> posix_writev()   -> __posix_writev() 
thread4: iot_worker -> ...  -> posix_writev()   |

there are 4 iot_worker thread doing the 128KB IO write fops as above, but 
only one can execute __posix_writev function and the others have to wait.

however, if the afr volume is configured on with storage.linux-aio which 
is off in default,  the iot_worker will use posix_aio_writev instead of 
posix_writev to write data.
the posix_aio_writev function won't be affected by 
GLUSTERFS_WRITE_IS_APPEND, and the AFR volume write performance goes up.

So, my question is whether  AFR volume could work fine with 
storage.linux-aio configuration which bypass the GLUSTERFS_WRITE_IS_APPEND 
setting in afr_writev, 
and why glusterfs keeps posix_aio_writev different from posix_writev ?

Any replies to clear my confusion would be grateful, and thanks in 
advance.
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