[Sheepdog] [PATCH 2/2] object cache: introduce async flush

[MORITA Kazutaka]

At Mon,  2 Apr 2012 16:21:11 +0800,
Liu Yuan wrote:
> 
> From: Liu Yuan <tailai.ly at taobao.com>
> 
> We async flush dirty object as default to achieve the best performance.
> If users prefer strong consistency over performance, users can launch
> sheep with -S or --sync option.
> 
> We need async flush because:
> 	1) some APP are responsive time sensitive, the writeback of dirty bits in
> 	the guest will mostly hurt RT because guest need to await its completion.
> 	This is a considerably long operation in the sheep cluster.
> 	2) some APP are just memory and CPU intensive, has little of concern of disk
> 	data. (For e.g, just use disk to store logs of APP)
> 	3) People simply prefer performance over consistency.

Sheepdog is a block device storage.  This kind of feature must NOT be
default.  In addition, we had better show a warning about a risk of
reading old data, which could cause a filesystem corruption, when
users enable this feature.

Kazutaka