Bug 243692 - file-backed VMs virtioblock report dev blocksize == underlying ZFS recordsize
Summary: file-backed VMs virtioblock report dev blocksize == underlying ZFS recordsize
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: bhyve (show other bugs)
Version: 12.1-RELEASE
Hardware: amd64 Any
: --- Affects Some People
Assignee: freebsd-virtualization (Nobody)
Depends on:
Reported: 2020-01-29 04:11 UTC by James Blachly
Modified: 2020-01-29 15:33 UTC (History)
0 users

See Also:


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Description James Blachly 2020-01-29 04:11:17 UTC
Briefly, file-backed virtio-block devices in bhyve are exposed differently than zvol backed VMs. This causes ZFS in illumos to segfault when a file-backed virtio-block device is used.

In illumos, the only difference in the block devices, as far as I can tell, is that zvol-backed devices have the ":lba-access-ok" flag set.

Possible cause:
The virtio block driver has a set of flags that expose supposed capabilities:


However, these differ between file-backed VMs and zvol-backed VMs, possibly here:


(if not specifically there, seems likely in that function, pci_vtblk_init)

This manifests in illumos as a segmentation fault when the command `zpool create` is issued on a bhyve-exposed virtio block device that is backed by a file.

I also filed an issue with illumos: https://www.illumos.org/issues/12237

Thanks in advance for help.
Comment 1 James Blachly 2020-01-29 15:32:32 UTC
On the illumos side, Hans Rosenfeld reports that the actual issue is that for file-backed virtio-block devices, bhyve reports the block size is equal to the recordsize of the underlying ZFS system, i.e. 128k in my case. This is larger than ASHIFT_MAX thus pool creation fails.

The :lba-access-ok flag is then unrelated but still of note.