We've been seeing a strange bug in KVM guests hosted by a Debian jessie box (running 3.16.7-ckt11-1+deb8u5 on x86-64),

Basically, we are getting random VirtIO errors inside our guests, resulting in stuff like this

[4735406.568235] blk_update_request: I/O error, dev vda, sector 142339584
[4735406.572008] EXT4-fs warning (device dm-0): ext4_end_bio:317: I/O error -5 writing to inode 1184437 (offset 0 size 208896 starting block 17729472)
[4735406.572008] Buffer I/O error on device dm-0, logical block 17729472
[ ... ]
[4735406.572008] Buffer I/O error on device dm-0, logical block 17729481
[4735406.643486] blk_update_request: I/O error, dev vda, sector 142356480
[ ... ]
[4735406.748456] blk_update_request: I/O error, dev vda, sector 38587480
[4735411.020309] Buffer I/O error on dev dm-0, logical block 12640808, lost sync page write
[4735411.055184] Aborting journal on device dm-0-8.
[4735411.056148] Buffer I/O error on dev dm-0, logical block 12615680, lost sync page write
[4735411.057626] JBD2: Error -5 detected when updating journal superblock for dm-0-8.
[4735411.057936] Buffer I/O error on dev dm-0, logical block 0, lost sync page write
[4735411.057946] EXT4-fs error (device dm-0): ext4_journal_check_start:56: Detected aborted journal
[4735411.057948] EXT4-fs (dm-0): Remounting filesystem read-only
[4735411.057949] EXT4-fs (dm-0): previous I/O error to superblock detected

(From an Ubuntu 15.04 guest, EXT4 on LVM2)

Or,

Jan 06 03:39:11 titanium kernel: end_request: I/O error, dev vda, sector 1592467904
Jan 06 03:39:11 titanium kernel: EXT4-fs warning (device vda3): ext4_end_bio:317: I/O error -5 writing to inode 31169653 (offset 0 size 0 starting block 199058492)
Jan 06 03:39:11 titanium kernel: Buffer I/O error on device vda3, logical block 198899256
[...]
Jan 06 03:39:12 titanium kernel: Aborting journal on device vda3-8.
Jan 06 03:39:12 titanium kernel: Buffer I/O error on device vda3, logical block 99647488

(From a Debian jessie guest, EXT4 directly on a VirtIO-based block device)

When this happens, it affects multiple guests on the hosts at the same time. Normally they are severe enough that they end up with a r/o file system, but we've seen a few hosts survive with a non-fatal I/O error. The host's dmesg has nothing interesting to see.

We've seen this happen with quite heterogeneous guests:

Debian 6, 7 and 8 (Debian kernels 2.6.32, 3.2 and 3.16)
Ubuntu 14.09 and 15.04 (Ubuntu kernels)
32 bit and 64 bit installs.

In short, we haven't seen a clear characteristic in any guest, other than the affected hosts being the ones with some sustained I/O load (build machines, cgit servers, PostgreSQL RDBMs...). Most of the times, hosts that just sit there doing nothing with their disks are not affected.

The host is a stock Debian jessie install that manages libvirt-based QEMU guests. All the guests have their block devices using virtio drivers, some of them on spinning media based on LSI RAID (was a 3ware card before, got replaced as we were very suspicious about it, but are getting the same results), and some of them based on PCIe SSD storage. We have some other 3 hosts, similar setup except they run Debian wheezy (and honestly we're not too keen on upgrading them yet, just in case), none of them has ever shown this kind of problem

We've been seeing this since last summer, and haven't found a pattern that tells us where these I/O error bugs are coming from. Google isn't revealing other people with a similar problem, and we're finding that quite surprising as our setup is quite basic.

This has also been reported downstream at the Debian BTS as Bug#810121 (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810121).