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Re: [dm-devel] dd to a striped device with 9 disks gets much lower throughput when oflag=direct used



Dne 27.1.2012 16:03, Richard Sharpe napsal(a):
On Fri, Jan 27, 2012 at 12:52 AM, Christoph Hellwig<hch infradead org>  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.


You've mentioned you are using thinp device with stripping - do you have
stripes properly aligned on data-block-size of thinp device ?
(I think 9 disks are properly quite hard to align somehow on 3.2 kernel,
since data block size needs to be power of 2 - I think 3.3 will have this
relaxed to page size boundary.

Zdenek



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