RE: [Xen-devel] Does Xen hypervisor overwrite O_DIRECT setting of Linux2.6 kernel?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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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