cache sync ioctl, freezes

[Greg Troxel <gdt%lexort.com@localhost>]

I am trying to track down zfs lockups -- which occur on systems without
xen, and I'm trying to repro in a domU.  Things are getting confusing
and I'm trying to run down all the bugs I find.

I have a dom0: netbsd-10 amd64, i7-12700.  system is ffs2, and there is
a zfs pool with one disk.

There are a bunch of zvols, basically two per domU, one for system
(often 32G) and one for swap (often 8G).   One has a third for zfs.
Created like so:

  zfs create -p -V 32G tank0/vm/n11-amd64
  zfs create -p -V 8G tank0/vm/n11-amd64.swap
  zfs create -p -V 32G tank0/vm/n11-amd64.zfs

Due to buggy code reported earlier, I have created workaround names using:

        rdsk_workaround ()
        {
            VOL=$1
            ln /dev/zvol/rdsk/tank0/vm/${VOL} /dev/zvol/dsk/tank0/vm/r${VOL}
        }

so that when you give the non-raw name in the config file, and it adds r
where it thinks, except that's wrong for zfs, it finds the raw file,
e.g. from

        disk = [
                'vdev=hda, target=/dev/zvol/dsk/tank0/vm/n11-amd64',
                'vdev=hdb, target=/dev/zvol/dsk/tank0/vm/n11-amd64.swap',
                'vdev=hdc, target=/dev/zvol/dsk/tank0/vm/n11-amd64.zfs'
                ]

Things are mostly ok, except:

  - I get spurious DIOCCACHESYNC failed messages (below).  I think
    that's because zvols don't support cache sync.  I don't know if this
    is harmless or not.

  - If I run the system out of ram (by mallocing 8GB in a system with
    6GB of RAM, and then writing to each page), the system freezes, by
    which I mean an ssh session stops responding to CR and if I do e.g
       echo 204500; date
    typing that at 204500, then the date printed is usually 2 minutes
    plus later.  (This happens without zfs loaded!)

  - When I run a script that creates 10000 files in zfs, it runs
    quickly, in seconds, maybe 10.  Trying to rm them takes over 5
    minutes.   Keep in mind that the disk in the zpool in the domU is
    backed by a zvol on the dom0.

So:

  - Does anyone else see the DIOCCACHESYNC error below?

  - Does anyone else see performance issues with  using a zvol for
    implementing a domU disk?



[ 702908.778860] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702940.499805] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 702940.729812] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 702948.050034] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702948.270040] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702958.510335] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702958.735652] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702968.960718] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702969.200666] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702970.960705] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 702971.190725] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 702988.451231] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 702988.685246] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703000.461588] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703000.691652] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703008.931835] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703009.171906] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703028.422424] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703028.642432] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703060.963392] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703061.203402] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703068.903633] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703069.143632] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703088.374215] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703088.604278] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703090.434337] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703090.674286] autoconfiguration error: xbdback backend/vbd/27/768: DIOCCACHESYNC returned 25
[ 703118.855115] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25
[ 703119.095191] autoconfiguration error: xbdback backend/vbd/24/768: DIOCCACHESYNC returned 25