[Sheepdog] [PATCH v6] add object cache for sheep

[Liu Yuan]

On 03/26/2012 09:48 PM, Liu Yuan wrote:

> NOTE:
>   You also need to patch following QEMU patch to issue flush requests and not all kernel will
> issue flush reqeust properly with upstream QEMU. For e.g, Debian squeeze will *not* issuse
> flush reqeust via qemu to sheepdog[Linux kernel has different implementation of flush req 
> and we works with new one]. 
> 
> Ubuntu server and RHEL 6 works as expected.
> 
> TODOs:
>  cache object quota and reclaim
> 
> v5 -> v6:
>  address Kazum's comments, Thanks!
>   - remove flock
>   - always read the cached object if any (solve qemu-img non-flush problem, collie vdi list. etc.)
>   - return err code to bdrv_co_flush_to_disk
>   - use valloc instead of malloc
>   - qemu patch v2
>     1 add cache flag for read_write_object()
>     2 adjust cache_enabled location in sd_open()
> 
> v4 -> v5:
>  - add control to disable cache
> 
> v3 -> v4:
>  - fix collie vdi list. Don't operate on cache for it.
>  - make flush operation from IO to LOCAL
> 
> v2 -> v3:
>  - refactored the object cache code, cache_object_fd is removed
>  - fix oc->dirty_rb race
>  - donot propagate error to guests for flush operation
>  - implement vdi delete operation
> 
> v1 -> v2:
>  - free entry when add_to_dirty_rb_and_list() fails.
>  - use mutex instead of spin lock.
> 
> 
>     Object cache caches data and vdi objects on the local node. It is at
> higher level than backend store. This extra cache layer translate gateway
> requests into local requests, largely reducing the network traffic and highly
> improving the IO performance.
>     
>     Dirty objects will be flushed to cluster storage by 'sync' request from
> guest OS.
> 
>     The initial version concentrate on the simplicity. Its design is straightforward
> just add one layer in front of the backend store. So the gateway requests (from Guest)
> 
> old path: IO req -> sheep cluster
>    |
>    v
> new path: IO req -> object cache on local node -> sheep cluster
> 
>     Later we might consider other features, for e.g., cache quota for different VDIs.
> 
> How To Use It:
> 
> The cache mode is controlled by the QEMU tool's DRIVE option,
> 
> To enable cache: qemu --enable-kvm -drive file=sheepdog:your_vm,cache=writeback
> To disable cache:
> 1 qemu --enable-kvm -drive file=sheepdog:your_vm,cache=none
> or
> 2 qemu --enable-kvm -drive file=sheepdog:your_vm


Applied.

Thanks,
Yuan