Description of problem:
Recent benchmarks on various environments have demonstrated that enabling Host-side disk cache can be beneficial for certain kind of workloads, specially read-mostly operations on a small data set, typical of DBs backing web services.
While QEMU already provides various Host-side disk cache modes, like writeback and writethrough, all of them are incompatible with live-migration.
Ideally, we should be able to make both writeback and writethrough. Alternatively, a safe way to switch between cache modes would allow the upper layers to coordinate the operation like this:
- (source) disable cache -> live-migrate -> (destination) enable cache
This feature is tricky and gets discussed from time to time in the QEMU community. I will raise it again upstream and see if there is willingness to accept it. The solution will probably have limitations but I'd like to get something merged.
Linux internally only has a best-effort API called invalidate_mapping_pages() which does not guarantee that cached pages are dropped. There are exceptions, like mmaped pages and in-progress Transparent Hugepages, which could lead to stale reads from the page cache on the destination host.
I have backported the QEMU patches necessary for shared storage live migration without cache=none.
Steps for verification:
1. Launch the guest on the migration source:
(src)# qemu-system-x86_64 ... -drive if=virtio,file=path/to/shared/test.img,format=raw,cache=writeback
2. Launch the guest on the migration destination:
(dst)# qemu-system-x86_64 ... -drive if=virtio,file=path/to/shared/test.img,format=raw,cache=writeback,file.x-check-cache-dropped=on -incoming tcp::1234
3. Start the migration:
(src)(qemu) migrate tcp:dst:1234
The live migration completes successfully.
The x-check-cache-dropped=on option validates that pages are not already in memory on the destination side after QEMU invalidates the cache. If this test fails you will see an error message.
This was moved from RHEL-7 to RHEL-8.
Patch that was sent is marked as rejected. So moving this back to assigned until is resent.
Stefan, I believe you will agree with this, right?