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



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.

Anyway, I now understand. I will run my scripts with conv=fdatasync as well.

-- 
Regards,
Richard Sharpe


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