On Fri, Jan 27, 2012 at 12:52 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Thu, Jan 26, 2012 at 05:06:42PM -0800, Richard Sharpe wrote: >> Why do I see such a big performance difference? Does writing to the >> device also use the page cache if I don't specify DIRECT IO? > > Yes. Trying adding conv=fdatasync to both versions to get more > realistic results. Thank you for that advice. I am comparing btrfs vs rolling my own thing using the new dm thin-provisioning approach to get something with resilient metadata, but I need to support two different types of IO, one that uses directio and one that can take advantage of the page cache. So far, btrfs gives me around 800MB/s with a similar setup (can't get exactly the same setup) without DIRECTIO and 450MB/s with DIRECTIO. a dm striped setup is giving me about 10% better throughput without DIRECTIO but only about 45% of the performance with DIRECTIO. Anyway, I now understand. I will run my scripts with conv=fdatasync as well. -- Regards, Richard Sharpe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
