[sheepdog] [RFC PATCH] object cache: revert object_cache_pull() to older version

[MORITA Kazutaka]

At Mon, 04 Jun 2012 14:12:10 +0800,
Liu Yuan wrote:
> 
> On 06/04/2012 02:04 PM, Liu Yuan wrote:
> 
> > The current object_cache_pull() cause bellow bug:
> > ...
> > do_gateway_request(288) 2, 80d6d76e00000000 , 1
> > Jun 04 10:16:37 connect_to(241) 2126, 10.232.134.3:7000
> > Jun 04 10:16:37 client_handler(747) closed connection 2116
> > Jun 04 10:16:37 destroy_client(678) connection from: 127.0.0.1:60214
> > Jun 04 10:16:37 listen_handler(797) accepted a new connection: 2116
> > Jun 04 10:16:37 client_rx_handler(586) connection from: 127.0.0.1:60216
> > Jun 04 10:16:37 queue_request(385) 2
> > Jun 04 10:16:37 do_gateway_request(288) 2, 80d6d76e00000000 , 1
> > Jun 04 10:16:37 do_gateway_request(308) failed: 2, 80d6d76e00000000 , 1, 54014b01
> > ...
> > 
> > This is because we use forward_read_obj_req(), which tries to multiplex a socket
> > FD if concurrent requests access to the same object and unforunately routed to
> > the same node.
> > 
> > Object cache has a very high pressure of current requests access to the same
> > COW object from cloned VMs, so this problem emerges. It looks to me that,
> > besides object cache, QEMU requests are  also be subject to this problem too
> > because QEMU's sheepdog block layer can issue multiple requests in one go.
> 
> 
> The alternative fix is to write a new fd cache, which allow mutiple FDs
> to the same node. This looks a better fix that sort out all the related
> problems

Can you explain how the current fd cache causes the above problem
against the concurrent accesses to the same node in more detail?

Thanks,

Kazutaka