[PATCH] bump up nr_to_write in xfs_vm_writepage

[Olaf Weber]

Eric Sandeen writes:

> Talking w/ someone who had a raid6 of 15 drives on an areca
> controller, he wondered why he could only get 300MB/s or so
> out of a streaming buffered write to xfs like so:

> dd if=/dev/zero of=/mnt/storage/10gbfile bs=128k count=81920
> 10737418240 bytes (11 GB) copied, 34.294 s, 313 MB/s

> when the same write directly to the device was going closer
> to 700MB/s...

> With the following change things get moving again for xfs:

> dd if=/dev/zero of=/mnt/storage/10gbfile bs=128k count=81920
> 10737418240 bytes (11 GB) copied, 16.2938 s, 659 MB/s

> Chris had sent out something similar at Christoph's suggestion,
> and Christoph reminded me of it, and I tested it a variant of
> it, and it seems to help shockingly well.

> Feels like a bandaid though; thoughts?  Other tests to do?

If the nr_to_write calculation really yields a value that is too
small, shouldn't it be fixed elsewhere?

Otherwise it might make sense to make the fudge factor tunable.

> +
> +	/*
> +	 *  VM calculation for nr_to_write seems off.  Bump it way
> +	 *  up, this gets simple streaming writes zippy again.
> +	 */
> +	wbc->nr_to_write *= 4;
> +

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