I'm testing setting up a Xen DomU with a DRBD storage for easy failover. Most of the time, immediatly after booting the DomU, I get an IO error:

[    3.153370] EXT3-fs (xvda2): using internal journal
[    3.277115] ip_tables: (C) 2000-2006 Netfilter Core Team
[    3.336014] nf_conntrack version 0.5.0 (3899 buckets, 15596 max)
[    3.515604] init: failsafe main process (397) killed by TERM signal
[    3.801589] blkfront: barrier: write xvda2 op failed
[    3.801597] blkfront: xvda2: barrier or flush: disabled
[    3.801611] end_request: I/O error, dev xvda2, sector 52171168
[    3.801630] end_request: I/O error, dev xvda2, sector 52171168
[    3.801642] Buffer I/O error on device xvda2, logical block 6521396
[    3.801652] lost page write due to I/O error on xvda2
[    3.801755] Aborting journal on device xvda2.
[    3.804415] EXT3-fs (xvda2): error: ext3_journal_start_sb: Detected aborted journal
[    3.804434] EXT3-fs (xvda2): error: remounting filesystem read-only
[    3.814754] journal commit I/O error
[    6.973831] init: udev-fallback-graphics main process (538) terminated with status 1
[    6.992267] init: plymouth-splash main process (546) terminated with status 1

The manpage of drbdsetup says that LVM (which I use) doesn't support barriers (better known as tagged command queuing or native command queing), so I configured the drbd device not to use barriers. This can be seen in /proc/drbd (by "wo:f, meaning flush, the next method drbd chooses after barrier):

 3: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r----
    ns:2160152 nr:520204 dw:2680344 dr:2678107 al:3549 bm:9183 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0

And on the other host:

 3: cs:Connected ro:Secondary/Primary ds:UpToDate/UpToDate C r----
    ns:0 nr:2160152 dw:2160152 dr:0 al:0 bm:8052 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0

I also enabled the option disable_sendpage, as per the drbd docs:

cat /sys/module/drbd/parameters/disable_sendpage
Y

I also tried adding barriers=0 to fstab as mount option. Still it sometimes says:

[   58.603896] blkfront: barrier: write xvda2 op failed
[   58.603903] blkfront: xvda2: barrier or flush: disabled

I don't even know if ext3 has a nobarrier option. And, because only one of my storage systems is battery backed, it would not be smart anyway.

Why does it still compain about barriers when I disabled that?

Both host are:

Debian: 6.0.4
uname -a: Linux 2.6.32-5-xen-amd64
drbd: 8.3.7
Xen: 4.0.1

Guest:

Ubuntu 12.04 LTS
uname -a: Linux 3.2.0-24-generic pvops

drbd resource:

resource drbdvm
{
  meta-disk internal;
  device /dev/drbd3;

  startup
  {
    # The timeout value when the last known state of the other side was available. 0 means infinite.
    wfc-timeout 0;

    # Timeout value when the last known state was disconnected. 0 means infinite.
    degr-wfc-timeout 180;
  }

  syncer
  {
    # This is recommended only for low-bandwidth lines, to only send those
    # blocks which really have changed.
    #csums-alg md5;

    # Set to about half your net speed
    rate 60M;

    # It seems that this option moved to the 'net' section in drbd 8.4. (later release than Debian has currently)
    verify-alg md5;
  }

  net
  {
    # The manpage says this is recommended only in pre-production (because of its performance), to determine
    # if your LAN card has a TCP checksum offloading bug.
    #data-integrity-alg md5;
  }

  disk
  {
    # Detach causes the device to work over-the-network-only after the
    # underlying disk fails. Detach is not default for historical reasons, but is
    # recommended by the docs.
    # However, the Debian defaults in drbd.conf suggest the machine will reboot in that event...
    on-io-error detach;

    # LVM doesn't support barriers, so disabling it. It will revert to flush. Check wo: in /proc/drbd. If you don't disable it, you get IO errors.
    no-disk-barrier;
  }

  on host1
  {
    # universe is a VG
    disk /dev/universe/drbdvm-disk;
    address 10.0.0.1:7792;
  }

  on host2
  {
    # universe is a VG
    disk /dev/universe/drbdvm-disk;
    address 10.0.0.2:7792;
  }
}

DomU cfg:

bootloader = '/usr/lib/xen-default/bin/pygrub'

vcpus       = '2'
memory      = '512'

#
#  Disk device(s).
#
root        = '/dev/xvda2 ro'
disk        = [
                  'phy:/dev/drbd3,xvda2,w',
                  'phy:/dev/universe/drbdvm-swap,xvda1,w',
              ]

#
#  Hostname
#
name        = 'drbdvm'

#
#  Networking
#
# fake IP for posting
vif         = [ 'ip=1.2.3.4,mac=00:16:3E:22:A8:A7' ]

#
#  Behaviour
#
on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'

In my test setup: the primary host's storage is 9650SE SATA-II RAID PCIe with battery. The secondary is software RAID1.

Isn't DRBD+Xen widely used? With these problems, it's not going to work.