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? 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? Thank you very much for your help! Best Wishes! Xiang ZHANG National Research Center for Intelligent Computing Systems Institute of Computing Technology, Chinese Academy of Sciences, Beijing, P.R.China _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Grzegorz Miłoś
2008-Jun-18 10:21 UTC
Re: [Xen-devel] How to test the disk performance in DU?
> 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. Cheers Gr(z)egor(z) _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel