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Re: [Xen-devel] How to test the disk performance in DU?

  • To: zhangxiang <zhangxiang@xxxxxxxxxx>
  • From: "Grzegorz Miłoś" <gm281@xxxxxxxxx>
  • Date: Wed, 18 Jun 2008 11:21:41 +0100
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 18 Jun 2008 03:22:04 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

> Hello, everybody:
> I have a question that how to test the disk performance in DU? In other
> word, how to eliminate the influence of D0's page cache?
> When using fsync() in DU, does Xen flush the dirty blocks into D0's page
> cache, or into the real disk?

There are two different I/O paths through Dom0: blkback or blktap.
blkback bypasses Dom0 page cache altogether, as it issues block I/O
ops directly.
blktap uses userspace daemons to handle DomU's I/O requests. There are
multiple drivers (as a conequence of supporting multiple disk image
formats), but they generally use O_DIRECT when accessing the image
files. So again, the Dom0's page cache should have little effect.

> When using direct I/O in DU, do the read and write operations bypass the
> DU's page cache, or bypass the D0's page cache?

Direct I/O (=O_DIRECT) in DomU should bypass both the DomU's (thanks
to linux kernel design) and Dom0's cache (as per explanation above).

BTW: The above holds for PV domains. I'm not certain if there is any
caching for HVM domains.


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