On 2/27/24 04:21, Vitaliy Gusev wrote:
Hi,
On 23 Feb 2024, at 18:37, Matthew Grooms <[email protected]> wrote:
...
The problem occurs when an image file is used on either ZFS or UFS.
The problem also occurs when the virtual disk is backed by a raw disk
partition or a ZVOL. This issue isn't related to a specific
underlying filesystem.
Do I understand right, you ran testing inside VM inside guest VM on
ext4 filesystem? If so you should be aware about additional overhead
in comparison when you were running tests on the hosts.
Hi Vitaliy,
I appreciate you providing the feedback and suggestions. I spent over a
week trying as many combinations of host and guest options as possible
to narrow this issue down to a specific host storage or a guest device
model option. Unfortunately the problem occurred with every combination
I tested while running Linux as the guest. Note, I only tested RHEL8 &
RHEL9 compatible distributions ( Alma & Rocky ). The problem did not
occur when I ran FreeBSD as the guest. The problem did not occur when I
ran KVM in the host and Linux as the guest.
I would suggest to run fio (or even dd) on raw disk device inside VM,
i.e. without filesystem at all. Just do not forget do “echo 3 >
/proc/sys/vm/drop_caches” in Linux Guest VM before you run tests.
The two servers I was using to test with are are no longer available.
However, I'll have two more identical servers arriving in the next week
or so. I'll try to run additional tests and report back here. I used
bonnie++ as that was easily installed from the package repos on all the
systems I tested.
Could you also give more information about:
1. What results did you get (decode bonnie++ output)?
If you look back at this email thread, there are many examples of
running bonnie++ on the guest. I first ran the tests on the host system
using Linux + ext4 and FreeBSD 14 + UFS & ZFS to get a baseline of
performance. Then I ran bonnie++ tests using bhyve as the hypervisor and
Linux & FreeBSD as the guest. The combination of host and guest storage
options included ...
1) block device + virtio blk
2) block device + nvme
3) UFS disk image + virtio blk
4) UFS disk image + nvme
5) ZFS disk image + virtio blk
6) ZFS disk image + nvme
7) ZVOL + virtio blk
8) ZVOL + nvme
In every instance, I observed the Linux guest disk IO often perform very
well for some time after the guest was first booted. Then the
performance of the guest would drop to a fraction of the original
performance. The benchmark test was run every 5 or 10 minutes in a cron
job. Sometimes the guest would perform well for up to an hour before
performance would drop off. Most of the time it would only perform well
for a few cycles ( 10 - 30 mins ) before performance would drop off. The
only way to restore the performance was to reboot the guest. Once I
determined that the problem was not specific to a particular host or
guest storage option, I switched my testing to only use a block device
as backing storage on the host to avoid hitting any system disk caches.
Here is the test script I used in the cron job ...
#!/bin/sh
FNAME='output.txt'
echo
================================================================================
>> $FNAME
echo Begin @ `/usr/bin/date` >> $FNAME
echo >> $FNAME
/usr/sbin/bonnie++ 2>&1 | /usr/bin/grep -v 'done\|,' >> $FNAME
echo >> $FNAME
echo End @ `/usr/bin/date` >> $FNAME
As you can see, I'm calling bonnie++ with the system defaults. That uses
a data set size that's 2x the guest RAM in an attempt to minimize the
effect of filesystem cache on results. Here is an example of the output
that bonnie++ produces ...
Version 2.00 ------Sequential Output------ --Sequential Input- --Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block--
--Seeks--
Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
linux-blk 63640M 694k 99 1.6g 99 737m 76 985k 99 1.3g 69
+++++ +++
Latency 11579us 535us 11889us 8597us 21819us 8238us
Version 2.00 ------Sequential Create------ --------Random
Create--------
linux-blk -Create-- --Read--- -Delete-- -Create-- --Read---
-Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
+++++ +++
Latency 7620us 126us 1648us 151us 15us 633us
--------------------------------- speed drop
---------------------------------
Version 2.00 ------Sequential Output------ --Sequential Input-
--Random-
-Per Chr- --Block-- -Rewrite- -Per Chr- --Block--
--Seeks--
Name:Size etc /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
linux-blk 63640M 676k 99 451m 99 314m 93 951k 99 402m 99
15167 530
Latency 11902us 8959us 24711us 10185us 20884us 5831us
Version 2.00 ------Sequential Create------ --------Random
Create--------
linux-blk -Create-- --Read--- -Delete-- -Create-- --Read---
-Delete--
files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
/sec %CP
16 0 96 +++++ +++ +++++ +++ 0 96 +++++
+++ 0 75
Latency 343us 165us 1636us 113us 55us 1836us
In the example above, the benchmark test repeated about 20 times with
results that were similar to the performance shown above the dotted line
( ~ 1.6g/s seq write and 1.3g/s seq read ). After that, the performance
dropped to what's shown below the dotted line which is less than 1/4 the
original speed ( ~ 451m/s seq write and 402m/s seq read ).
2. What results expecting?
What I expect is that, when I perform the same test with the same
parameters, the results would stay more or less consistent over
time. This is true when KVM is used as the hypervisor on the same
hardware and guest options. That said, I'm not worried about bhyve being
consistently slower than kvm or a FreeBSD guest being consistently
slower than a Linux guest. I'm concerned that the performance drop over
time is indicative of an issue with how bhyve interacts with non-freebsd
guests.
3. VM configuration, virtio-blk disk size, etc.
4. Full command for tests (including size of test-set), bhyve, etc.
I believe this was answered above. Please let me know if you have
additional questions.
5. Did you pass virtio-blk as 512 or 4K ? If 512, probably you should
try 4K.
The testing performed was not exclusively with virtio-blk.
6. Linux has several read-ahead options for IO schedule, and it could
be related too.
I suppose it's possible that bhyve could be somehow causing the disk
scheduler in the Linux guest to act differently. I'll see if I can
figure out how to disable that in future tests.
Additionally could also you play with “sync=disabled” volume/zvol
option? Of course it is only for write testing.
The testing performed was not exclusively with zvols.
Once I have more hardware available, I'll try to report back with more
testing. It may be interesting to also see how a Windows guest performs
compared to Linux & FreeBSD. I suspect that this issue may only be
triggered when a fast disk array is in use on the host. My tests use a
16x SSD RAID 10 array. It's also quite possible that the disk IO
slowdown is only a symptom of another issue that's triggered by the disk
IO test ( please see end of my last post related to scheduler priority
observations ). All I can say for sure is that ...
1) There is a problem and it's reproducible across multiple hosts
2) It affects RHEL8 & RHEL9 guests but not FreeBSD guests
3) It is not specific to any host or guest storage option
Thanks,
-Matthew
