[sheepdog] [Sheepdog] [PATCH v2] sheep: update inode cache first

[Liu Yuan]

On 05/16/2012 12:04 PM, Christoph Hellwig wrote:

> On Wed, May 16, 2012 at 12:05:51PM +0900, MORITA Kazutaka wrote:
>>> > > flushing the cache, we already have mechanism to to do this, qemu-io -c
>>> > > "flush".
>> > 
>> > IMHO, the uppler layer should assume that the underlying one provides
>> > a block storage semantic to simplify the implementation.  And
>> > actually, there is no other block driver in QEMU and OpenStack which
>> > needs a flush request to read the latest data from another node.
> Actually there is in QEMU - it's file backed images on NFS for certain
> coner cases, and they are a massive PITA to handle, last time I checked
> with Redhat-private patches not even in upstream qemu for live
> migration.
> 


Even there isn't any block driver in QEMU issuing flush before reading
for any corner case. I will insist on fixing the problem in the upper
layer because (quote from 'Worse is Better'):

"Simplicity-the design must be simple, both in implementation and
interface. It is more important for the implementation to be simple than
the interface. Simplicity is the most important consideration in a design. "

So my point is, interface to upper layer should adopt to the lower level
implementation, *if* we have a rationale behind. In our case,
restriction in upper layer(be it adding a flush before reading, use
correct cache mode for snapshot.etc.) will benefit us

1) the problem we have for now will disappear completely
2) greatly reduce the internal implementation logic

But anyway, if it is not easy to fix upper layer or want to get most
backward compatible, it would be better to try our best effort (with
minimal cost) to handle(reduce) the problem in the lower implementation.

Thanks,
Yuan