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Hi,<br>
<br>
Lars Ellenberg wrote:
<blockquote cite="mid:20090216184628.GB12755@barkeeper1-xen.linbit"
type="cite">
<pre wrap="">On Mon, Feb 16, 2009 at 10:51:45AM +0000, Igor Neves wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Hi,
Some of you may know my problem since I have been discussing it with you
in the IRC in the last few days, but I will explain it.
I'm doing some tests with drbd with xen virtual machines, our solution
was with vmware and drbd, but this solution it's with very low
performance compared to xen, and i'm trying to improve our solution to xen.
I started my tests with xen on both machines (node1 & node2). Both
machines have 3 disks, sdb it's the drbd0.
The disk can only deliver 102MB/sec writing, following seagate, and I
get about 105MB/sec from each disk in direct access.
I started my resouce in drbd and i'm able to sync at about 95MB/sec,
which it's very good. Testing the disk with dd or with your benchmark
tool called md, I get about 85MB/sec writing, which it's not bad.
Now when I start the Xen Windows HVM Virtual Machine on node1 and I give
"/dev/drbd0" as a block device, it works great, but inside windows VM, I
get:
- With drbd rpm package 8.2.6 from redhat and with this configuration
"al-extents 1801" i get about 45MB/sec writing inside VM.
- With drbd rpm 8.2.7 and 8.3.0 from linbit and with this configuration
"al_extents 1801" i get about 4MB/sec writing inside VM.
- With source 8.2.7 and 8.3.0 from linbit and with "al_extents 1801" i
get again 4MB/sec writing inside VM.
Now what I have found it's, all this tests were made with drbd in
connected & updated state. In the middle of the test if I do in node2
"drbdadm disconnect resource" I get the state WForconnection and Update
on node1 and in that moment I start getting 85MB/sec.
So inside Xen Windows HVM Virtual Machine in and with drbd under it as a
block device i get:
- drbd 8.2.6 from redhat and drbd connected = 45MB/sec, with drbd
disconnected = 85MB/sec
- drbd 8.2.7 from linbit and drbd connected = 4MB/sec, with drbd
disconnected = 85MB/sec
- drbd 8.3.0 from linbit and drbd connected = 4MB/sec, with drbd
disconnected = 85MB/sec
</pre>
</blockquote>
<pre wrap=""><!---->
in 8.2.7, we introduced a new method to ensure write ordering
guarantees, which now needs to be explicitly disabled
if you live on a "safe" device (battery backed write cache,
or no cache at all), where this hurts performance.
so where you had "no-disk-flushes" in drbd.conf before,
you now need an additional "no-disk-barrier".
if that does not help, or if you did not have "no-disk-flushes" before,
let me know, and I try to come up with something else.
</pre>
</blockquote>
<br>
Ok, I have added "no-disk-barrier" and now I get about 50MB/sec inside
the VM.<br>
<br>
Outside the VM, in host, I get about 85MB/sec and with resource
disconnected I get 85MB/sec.<br>
<br>
It's there something we can do to get this 25MB/sec difference?<br>
<br>
<blockquote cite="mid:20090216184628.GB12755@barkeeper1-xen.linbit"
type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">I know you can ask, it's not Xen block device driver?
- Yes it could, so i have added the other disk (exacly equal to the one
in drbd resource and with the same throughput) with direct access and
inside THE SAME windows virtual machine, I have 100MB/sec writing to the
disk, so the Xen block device it's going good and it's working like a
charm. Besides that Xen block device does not know "connected" and
"disconnected" as I explained in the early example.
I have done this tests all again in node2 as Brian suggested, but i get
exactly the same values in the tests.
So there is a problem here with drbd<->xen, something I have not managed
to find and could be a bug or maybe some bad configuration from my side.
Please, someone with more experience help out on this.
My kernel version it's "2.6.18-92.1.22.el5xen" and I'm with Centos 5 and
with all the latest updates available.
Thanks,
Igor Neves
</pre>
</blockquote>
</blockquote>
<br>
Thanks,<br>
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