[ovirt-users] VMs becoming non-responsive sporadically

Fri Apr 29 21:34:45 UTC 2016

On Fri, Apr 29, 2016 at 9:17 PM,  <nicolas at devels.es> wrote:
> Hi,
>
> We're running oVirt 3.6.5.3-1 and lately we're experiencing some issues with
> some VMs being paused because they're marked as non-responsive. Mostly,
> after a few seconds they recover, but we want to debug precisely this
> problem so we can fix it consistently.
>
> Our scenario is the following:
>
> ~495 VMs, of which ~120 are constantly up
> 3 datastores, all of them iSCSI-based:
>   * ds1: 2T, currently has 276 disks
>   * ds2: 2T, currently has 179 disks
>   * ds3: 500G, currently has 65 disks
> 7 hosts: All have mostly the same hardware. CPU and memory are currently
> very lowly used (< 10%).
>
>   ds1 and ds2 are physically the same backend which exports two 2TB volumes.
> ds3 is a different storage backend where we're currently migrating some
> disks from ds1 and ds2.

What the the storage backend behind ds1 and 2?

>
> Usually, when VMs become unresponsive, the whole host where they run gets
> unresponsive too, so that gives a hint about the problem, my bet is the
> culprit is somewhere on the host side and not on the VMs side.

Probably the vm became unresponsive because connection to the host was lost.

> When that
> happens, the host itself gets non-responsive and only recoverable after
> reboot, since it's unable to reconnect.

Piotr, can you check engine log and explain why host is not reconnected?

> I must say this is not specific to
> this oVirt version, when we were using v.3.6.4 the same happened, and it's
> also worthy mentioning we've not done any configuration changes and
> everything had been working quite well for a long time.
>
> We were monitoring our ds1 and ds2 physical backend to see performance and
> we suspect we've run out of IOPS since we're reaching the maximum specified
> by the manufacturer, probably at certain times the host cannot perform a
> storage operation within some time limit and it marks VMs as unresponsive.
> That's why we've set up ds3 and we're migrating ds1 and ds2 to ds3. When we
> run out of space on ds3 we'll create more smaller volumes to keep migrating.
>
> On the host side, when this happens, we've run repoplot on the vdsm log and
> I'm attaching the result. Clearly there's a *huge* LVM response time (~30
> secs.).

Indeed the log show very slow vgck and vgs commands - these are called every
5 minutes for checking the vg health and refreshing vdsm lvm cache.

1. starting vgck

Thread-96::DEBUG::2016-04-29
13:17:48,682::lvm::290::Storage.Misc.excCmd::(cmd) /usr/bin/taskset
--cpu-list 0-23 /usr/bin/sudo -n /usr/sbin/lvm vgck --config ' devices
{ pre
ferred_names = ["^/dev/mapper/"] ignore_suspended_devices=1
write_cache_state=0 disable_after_error_count=3 filter = [
'\''a|/dev/mapper/36000eb3a4f1acbc20000000000000043|'\
'', '\''r|.*|'\'' ] }  global {  locking_type=1
prioritise_write_locks=1  wait_for_locks=1  use_lvmetad=0 }  backup {
retain_min = 50  retain_days = 0 } ' 5de4a000-a9c4-48
9c-8eee-10368647c413 (cwd None)

2. vgck ends after 55 seconds

Thread-96::DEBUG::2016-04-29
13:18:43,173::lvm::290::Storage.Misc.excCmd::(cmd) SUCCESS: <err> = '
WARNING: lvmetad is running but disabled. Restart lvmetad before
enabling it!\n'; <rc> = 0

3. starting vgs

Thread-96::DEBUG::2016-04-29
13:17:11,963::lvm::290::Storage.Misc.excCmd::(cmd) /usr/bin/taskset
--cpu-list 0-23 /usr/bin/sudo -n /usr/sbin/lvm vgs --config ' devices
{ pref
erred_names = ["^/dev/mapper/"] ignore_suspended_devices=1
write_cache_state=0 disable_after_error_count=3 filter = [
'\''a|/dev/mapper/36000eb3a4f1acbc20000000000000043|/de
v/mapper/36000eb3a4f1acbc200000000000000b9|/dev/mapper/360014056f0dc8930d744f83af8ddc709|/dev/mapper/WDC_WD5003ABYZ-011FA0_WD-WMAYP0J73DU6|'\'',
'\''r|.*|'\'' ] }  global {
 locking_type=1  prioritise_write_locks=1  wait_for_locks=1
use_lvmetad=0 }  backup {  retain_min = 50  retain_days = 0 } '
--noheadings --units b --nosuffix --separator '|
' --ignoreskippedcluster -o
uuid,name,attr,size,free,extent_size,extent_count,free_count,tags,vg_mda_size,vg_mda_free,lv_count,pv_count,pv_name
5de4a000-a9c4-489c-8eee-10368
647c413 (cwd None)

4. vgs finished after 37 seconds

Thread-96::DEBUG::2016-04-29
13:17:48,680::lvm::290::Storage.Misc.excCmd::(cmd) SUCCESS: <err> = '
WARNING: lvmetad is running but disabled. Restart lvmetad before
enabling it!\n'; <rc> = 0

Zdenek, how do you suggest to debug this slow lvm commands?

Can you run the following commands on a host in trouble, and on some other
hosts in the same timeframe?

time vgck -vvvv --config ' devices { filter =
['\''a|/dev/mapper/36000eb3a4f1acbc20000000000000043|'\'',
'\''r|.*|'\'' ] }  global {  locking_type=1  prioritise_write_locks=1
wait_for_locks=1  use_lvmetad=0 }  backup {  retain_min = 50
retain_days = 0 } ' 5de4a000-a9c4-489c-8eee-10368647c413

time vgs -vvvv --config ' global { locking_type=1
prioritise_write_locks=1  wait_for_locks=1  use_lvmetad=0 }  backup {
retain_min = 50  retain_days = 0 } '
5de4a000-a9c4-489c-8eee-10368647c413

Note that I added -vvvv to both commands, this will generate huge amount
of debugging info. Please share the output of these commands.

You may need to fix the commands. You can always copy and paste directly
from vdsm log into the shell and add the -vvvv flag.

> Our host storage network is correctly configured and on a 1G
> interface, no errors on the host itself, switches, etc.

1G serving about 20-70 vms per host? (495 vms, 120 always up, 7 hosts)?

Do you have separate network for management and storage, or both
use this 1G interface?

> We've also limited storage in QoS to use 10MB/s and 40 IOPS,

How did you configure this limit?

> but this issue
> still happens, which leads me to be concerned whether this is not just an
> IOPS issue; each host handles about cca. 600 LVs. Could this be an issue
> too? I remark the LVM response times are low in normal conditions (~1-2
> seconds).

We recommend to limit number lvs per vg to 350. If you have 276 disks on
ds1, and the disks are using snapshots, you may have too many lvs, which
can cause slowdowns in lvm operations.

Can you share the output of:

vdsm-tool dump-volume-chains 5de4a000-a9c4-489c-8eee-10368647c413

> I'm attaching the vdsm.log, engine.log and repoplot PDF;

This is very useful, thanks. Can you send also the vdsm logs and repoplots
from other hosts for the same timeframe?

> if someone could
> give a hint on some additional problems in them and shed some light on the
> above thoughts I'd be very grateful.

Do you have sar configured on the host? having sar logs can reveal more
info about this timeframe.

Do you have information about amount of io from vms during this timeframe?
amount of io on the storage backend during this timeframe?

Nir