Hi, I have a very curious problem on an oVirt-like virtualization host whose storage lives on gluster (as qcow2).
The problem is that of the writes done by ZFS, whose sizes according to blktrace are a mixture of 8, 16, 24, ... 256 (512 byte) blocks, sometimes the first 4KB or more, but at least the first 4KB, end up zeroed out when read back later from storage. In my current test scenario, I write approx. 3GB to the guest machine, which takes roughly a minute. Actually it’s around 35 GB which gets compressed down to 3GB by lz4. Within that, I end up with close to 100 data errors when I read it back from storage afterwards (zpool scrub). There are quite a few machines running on this host, and we have not experienced other problems so far. So right now, only ZFS is able to trigger this for some reason. The guest has 8 virtual cores. I also tried writing directly to the affected device from user space in patterns mimicking what I see in blktrace, but so far have been unable to trigger the same issue that way. Of the many ZFS knobs, I know at least one that makes a huge difference: When I set zfs_vdev_async_write_max_active to 1 (as opposed to 2 or 10), the error count goes through the roof (11.000). Curiously, when I switch off ZFS compression, the data amount written increases almost 10-fold, while the absolute error amount drops to close to, but not entirely, zero. Which I guess supports my suspicion that this must be somehow related to timing. Switching the guest storage driver between scsi and virtio does not make a difference. Switching the storage backend to file on glusterfs-fuse does make a difference, i.e. the problem disappears. Any hints? I'm still trying to investigate a few things, but what bugs me most that only ZFS seems to trigger this behavior, although I am almost sure that ZFS is not really at fault here. Software versions used: Host kernel 3.10.0-957.12.1.el7.x86_64 qemu-kvm-ev-2.12.0-18.el7_6.3.1.x86_64 glusterfs-api-5.6-1.el7.x86_64 Guest kernel 3.10.0-1062.12.1.el7.x86_64 kmod-zfs-0.8.3-1.el7.x86_64 (from the official ZoL binaries)
