On 03/26/2012 01:39 PM, MORITA Kazutaka wrote: > At Mon, 26 Mar 2012 12:15:29 +0800, > Liu Yuan wrote: >> >> On 03/26/2012 04:41 AM, MORITA Kazutaka wrote: >> >>> Consider that the object is dirty if the VDI was opened with a >>> writeback mode in the previous time. I think we need to check a cache >>> here. >> >> >> For the sane usage, VMs should not be opened both for none-cache and >> cache mode off and on. Even if VM first open the vdi with cache mode >> first, then want to use none-cache mode, current implementation would be >> okay, since after the guest was closed, the dirty bits would be flushed >> to the cluster. >> >> So the problem is precisely that we can't handle the situation that VM >> runs with cache first, then crashes, then boots up again without cache. >> We'd better use qemu-io to issue a flush requests first manually, then >> we can safe boot the VM without cache, not losing any dirty data. >> >> So I don't think it worth introducing complexity to object cache to >> handle this corner case that already should be handled by other means. > > We can more easily hit this problem. For example: > > $ qemu-img convert linux.raw sheepdog:linux > > where linux.raw is a linux image file, and > > $ qemu sheepdog:linux > > We cannot boot the linux VDI because qemu-img opens with a > BDRV_O_CACHE_WB always, but qemu opens without it by default. >
I think this is more a bug of qemu-img than of object cache. We should fix it at qemu-img level. If it is default to open the file as BDRV_O_CACHE_WB, its qemu-img's responsibility to flush the dirty bits. As a temporary fix, we can use qemu-io -c "flush" sheepdog:linux before launch the VM. > I think you are mixing two things in this patchset. > 1. use a disk cache (this should be disabled by BDRV_O_NOCACHE) > 2. support a writeback mode (this should be enabled by BDRV_O_CACHE_WB) > This depends how you define what disk cache does. I don't follow qemu's definition of cache mode(none, write through, write back), and I think it simply complicates the usage and code. Even http://www.linux-kvm.org/page/Tuning_KVM suggests that use cache=none mode without convincing explanation. So for object cache, the notion is simple, none or have it. cache=writeback is somewhat misleading, would be better scripted as 'cache=on', but I can't find any better flag to activate object cache from qemu. > If you think it doesn't make sense to support a writethrough mode with > a disk cache, I'm not against to the current approach in this > patchset. But at least, please flush a dirty cache before handling > non-cache requests. Otherwise, we can easily hit a cache coherency > problem (e.g. 'collie vdi list' doesn't show the correct used size). > Yes, for vdi operation, we need flush first. Thanks, Yuan -- sheepdog mailing list [email protected] http://lists.wpkg.org/mailman/listinfo/sheepdog
