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Subject: Re: [virtio] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks
[ Moved to virtio-dev, since we're now talking about an implementation ] Paolo Bonzini <email@example.com> writes: > Il 17/09/2013 03:20, Rusty Russell ha scritto: >> bdrv_co_flush() calls: >> >> bs->drv->bdrv_co_flush_to_disk >> >> or if that's NULL: >> >> bs->drv->bdrv_aio_flush >> >> or if that's NULL, does nothing. >> >> Now, qcow2 doesn't set bdrv_co_flush_to_disk or bdrv_aio_flush. > > It also does > > bdrv_co_flush(bs->file); > > and that's where bdrv_co_flush_to_disk gets called. > >> It seems a raw file will get an fdatasync, via bdrv_aio_flush() -> >> raw_aio_flush(), but strace shows no fsync/fdatasync calls, even when I >> just a raw file, and type "sync" in the guest: > > That's the same path that gets qcow2 flushed, actually. You need > "strace -ff" because the flush is done in a thread pool. Oops, I missed the -f. Unfortunately, it doesn't help. I only get fdatasync on shutdown, not when I do ops in the guest. So I ended up tracking this into the virtio_blk driver: the guest isn't submitting any flush commands. It's accepted both the VIRTIO_BLK_F_WCE and VIRTIO_BLK_F_CONFIG_WCE flags, and /sys/devices/pci0000:00/0000:00:04.0/virtio0/block/vda/cache_type says "write back". It's mounted using ext4, default options. 3.12-rc1. Clearly, I'm missing some Linux block layer thing. How do I get it to submit a flush to the virtio_blk driver? I would expect "touch some-new-file; sync" to do that. Seriously confused, Rusty.