Hi Ian,
Thanks a lot for your prompt reply.
I'm doing physical drive performance testing for SAS drives. All the SAS
drives under test are indeed raw drives (no filesystems on it). Each SAS
drive is assigned maximum 32 outstanding I/Os. Linux, Xen and benchmark tool
IOMeter are installed on a separate SATA drive. I don't know how disk zoning
could be related to this issue.
Only for sequential write of small packets (512B and 1KB), Xen domain0
outperform Linux native. For sequential/random read (512B, 1K, 4K, 8K, 64K,
128K and 256K), Linux native outperform Xen domain0(this is expected). For
random write(4K and 8K), both Xen domain0 and Linux native have almost
identical performance.
Liang
----- Original Message -----
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>;
<xen-devel@xxxxxxxxxxxxxxxxxxx>
Cc: <ian.pratt@xxxxxxxxxxxx>
Sent: Saturday, October 28, 2006 3:35 PM
Subject: RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of
Linux2.6 kernel?
Yes, all the testing configurations are the exactly same (both uses
Linux
kernel 2.6.16.29) and I used Xen 3.0.3. I did several indepedent runs
for
my testing and the results are consistent, e.g. the performance of
small
packet
sequential write under Xen Domain0 outperform Linux native by 10~20%.
Some people told me Xen hypervisor adds addtional layer beyond Linux
I/O
stack. So O_DIRECT under Xen hypervisor is still valid, but I/O
coalescing
for small packets are enabled by Xen hypervisor. However, Linux native
kernel would not do any I/O coalescing when using O_DIRECT.
For IO from dom0 there is no additional layer. There will be no
difference in the coalescing behaviour.
Are you sure you're writing to the same part of the disk in both cases?
You know about disk zoning, right?
Ian
I hope Xen gurus can give me more explanations about this.
Thanks,
Liang
----- Original Message -----
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
To: "Liang Yang" <multisyncfe991@xxxxxxxxxxx>;
<xen-devel@xxxxxxxxxxxxxxxxxxx>
Cc: <ian.pratt@xxxxxxxxxxxx>
Sent: Saturday, October 28, 2006 2:34 PM
Subject: RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT
setting of
Linux2.6 kernel?
> The performance data is collected on 8 SAS drives (used as physical
drives)
> and IOMeter is used as the benchmark tool. The latest IOMeter
version
used
> O_DIRECT. We know, Linux 2.6 kernel starts supporting O_DIRECT which
makes
> all I/O requests work around buffer cache. The good thing for
O_DIRECT
is
> it
> reduces the CPU utilization and cache pollution. The bad thing is
O_DIRECT
> not only forces all I/O requests become synchronous and no I/O
coalescing
> will happen. Thus sequential write of small packets will be impacted
most.
> For Xen, however, I believe Xen hypervisor overwrites this O_DIRECT
setting
> and maybe it favors better performance over CPU and FSB utilization.
Thus
> Xen domain0 can have better write performance than Linux native.
Are you sure you're comparing identical native and dom0 kernel
versions?
Same drivers and settings?
Xen does not disable O_DIRECT.
Ian
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