6:16 a.m.
Hi Eric, a couple of questions below about nbdkit performance.
Modular virt-v2v will use disk pipelines everywhere. The input
pipeline looks something like this:
socket <- cow filter <- cache filter <- nbdkit
curl|vddk
We found there's a notable slow down in at least one case: When the
source plugin is very slow (eg. it's curl plugin to a slow and remote
website, or VDDK in general), everything runs very slowly.
I made a simple test case to demonstrate this:
$ virt-builder fedora-33
$ time ./nbdkit --filter=cache --filter=delay file /var/tmp/fedora-33.img delay-read=500ms
--run 'virt-inspector --format=raw -a "$uri" -vx'
This uses a local file with the delay filter on top injecting half
second delays into every read. It "feels" a lot like the slow case we
were observing. Virt-v2v also does inspection as a first step when
converting an image, so using virt-inspector is somewhat realistic.
Unfortunately this actually runs far too slowly for me to wait around
- at least 30 mins, and probably a lot longer. This compares to only
7 seconds if you remove the delay filter.
Reducing the delay to 50ms means at least it finishes in a reasonable time:
$ time ./nbdkit --filter=cache --filter=delay file /var/tmp/fedora-33.img \
delay-read=50ms \
--run 'virt-inspector --format=raw -a "$uri"'
real 5m16.298s
user 0m0.509s
sys 0m2.894s
In the above scenario the cache filter is not actually doing anything
(since virt-inspector does not write). Adding cache-on-read=true lets
us cache the reads, avoiding going through the "slow" plugin in many
cases, and the result is a lot better:
$ time ./nbdkit --filter=cache --filter=delay file /var/tmp/fedora-33.img \
delay-read=50ms cache-on-read=true \
--run 'virt-inspector --format=raw -a "$uri"'
real 0m27.731s
user 0m0.304s
sys 0m1.771s
However this is still slower than the old method which used qcow2 +
qemu's copy-on-read. It's harder to demonstrate this, but I modified
virt-inspector to use the copy-on-read setting (which it doesn't do
normally). On top of nbdkit with 50ms delay and no other filters:
qemu + copy-on-read backed by nbdkit delay-read=50ms file:
real 0m23.251s
So 23s is the time to beat. (I believe that with longer delays, the
gap between qemu and nbdkit increases in favour of qemu.)
Q1: What other ideas could we explore to improve performance?
- - -
In real scenarios we'll actually want to combine cow + cache, where
cow is caching writes, and cache is caching reads.
socket <- cow filter <- cache filter <- nbdkit
cache-on-read=true curl|vddk
The cow filter is necessary to prevent changes being written back to
the pristine source image.
This is actually surprisingly efficient, making no noticable
difference in this test:
time ./nbdkit --filter=cow --filter=cache --filter=delay \
file /var/tmp/fedora-33.img \
delay-read=50ms cache-on-read=true \
--run 'virt-inspector --format=raw -a "$uri"'
real 0m27.193s
user 0m0.283s
sys 0m1.776s
Q2: Should we consider a "cow-on-read" flag to the cow filter (thus
removing the need to use the cache filter at all)?
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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