[sheepdog] Is it necessary for outstanding io block leave/join event?

[Liu Yuan]

On 05/17/2012 03:29 PM, MORITA Kazutaka wrote:

>> > 
>> > This assumption seems not necessary, at least to Farm, where I/O will
>> > always be routed into objects in the working directory.
> Really?  I thought that this problem does not depend on the underlying
> storage driver.
> 
> If there are 1 node, A, and the number of copies is 1, how does
> Farm handle the following case?
> 
>  - the user add the second node B, and there is in-flight I/Os on
>    node A
>  - the node A increments the epoch from 1 to 2, and the node B recovers
>    objects from epoch 1 on node A
>  - after node B receives objects to epoch 2, the in-flight I/Os on
>    node A updates objects in epoch 1 on node A.


This is really a race problem for Farm for now. But I think we can
exclude it by:

1) ask recovery request in A to check if the requested oid is on the
outstanding list.
2) if yes, A put it on a waiting list, if no, service the requests
3) the in-fly IO finished on A, check if there is any request on waiting
list, if yes, resume it.

I think this algorithm will allow us a finer blocking for the request
who really need blocking. Our current algo will block all the requests,
most of them will be poor victim.

Thanks,
Yuan