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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 16:28, Richard Sharpe napsal(a):
On Fri, Jan 27, 2012 at 7:16 AM, Zdenek Kabelac<zkabelac redhat com>  wrote:
Dne 27.1.2012 16:03, Richard Sharpe napsal(a):

On Fri, Jan 27, 2012 at 12:52 AM, Christoph Hellwig<hch infradead org>

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

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.

Actually, so far I have not used any thinp devices, since from reading
the documentation it seemed that, for what I am doing, I need to give
thinp a mirrored device for its metadata and a striped device for its
data, so I thought I would try just a striped device.

Actually, I can cut that back to 8 devices in the stripe. I am using
4kiB block sizes and writing 256kiB blocks in the dd requests and
there is no parity involved so there should be no read-modify-write

I imagine that if I push the write sizes up to a MB or more at a time
throughput will get better because at the moment each device is being
given 32kIB or 16kiB (a few devices) with DIRECTIO and with a larger
write size they will get more data at a time.

Well I cannot tell how big influence proper alignment has in your case, but it would be good to measure it in your case.
Do you use data_block_size equal to stripe size (256KiB 512blocks ?)


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