svn commit: r223144 - projects/hastd_wan

[Pawel Jakub Dawidek]

On Thu, Jun 16, 2011 at 08:44:26AM +0000, Maxim Sobolev wrote:
> Author: sobomax
> Date: Thu Jun 16 08:44:26 2011
> New Revision: 223144
> URL: http://svn.freebsd.org/changeset/base/223144
> 
> Log:
>   "HAST currently doesn't support 'async' mode, so working over WAN is not
>    common use case for now, which makes your changes low priority. High
>    priority for HAST is working over LAN." -pjd
>   
>   So this little fork of the hastd would aim at optimizing it for good
>   performance over WAN network. Hopefully it will be useful for somebody
>   else and and end up at least partially in the main tree.

Let me share my ideas how I think 'async' should be implemented.

In 'async' mode we send writes to the remote node, but don't wait for
confirmation. This way we can complete the write much faster of course,
but we still need to keep the extents dirty until remote node confirms
that write succeeded.
Then you need to limit the number of outstanding 'async' writes as we
can easly consume huge amount of memory and make the activemap very
dirty. Which shows that 'async' will only work for configuration where
you have relatively short write peaks.
Once that done you may think about combining outstanding I/Os and
removing covered parts of earlier outstanding I/Os.

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://yomoli.com
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