Re: [PATCH v4 01/11] block: make generic_make_request handle arbitrarily sized bios

[Ming Lin <mlin@kernel.org>]

On Thu, 2015-05-28 at 01:36 +0100, Alasdair G Kergon wrote:
> On Wed, May 27, 2015 at 04:42:44PM -0700, Ming Lin wrote:
> > Here are fio results of XFS on a DM stripped target with 2 SSDs + 1 HDD.
> > Does it make sense?
> 
> To stripe across devices with different characteristics?
> 
> Some suggestions.
> 
> Prepare 3 kernels.
>   O - Old kernel.
>   M - Old kernel with merge_bvec_fn disabled.
>   N - New kernel.
> 
> You're trying to search for counter-examples to the hypothesis that 
> "Kernel N always outperforms Kernel O".  Then if you find any, trying 
> to show either that the performance impediment is small enough that
> it doesn't matter or that the cases are sufficiently rare or obscure
> that they may be ignored because of the greater benefits of N in much more
> common cases.
> 
> (1) You're looking to set up configurations where kernel O performs noticeably
> better than M.  Then you're comparing the performance of O and N in those
> situations.
> 
> (2) You're looking at other sensible configurations where O and M have
> similar performance, and comparing that with the performance of N.

I didn't find case (1).

But the important thing for this series is to simplify block layer
based on immutable biovecs. I don't expect performance improvement.

Here is the changes statistics.

"68 files changed, 336 insertions(+), 1331 deletions(-)"

I run below 3 test cases to make sure it didn't bring any regressions.
Test environment: 2 NVMe drives on 2 sockets server.
Each case run for 30 minutes.

2) btrfs radi0

mkfs.btrfs -f -d raid0 /dev/nvme0n1 /dev/nvme1n1
mount /dev/nvme0n1 /mnt

Then run 8K read.

[global]
ioengine=libaio
iodepth=64
direct=1
runtime=1800
time_based
group_reporting
numjobs=4
rw=read

[job1]
bs=8K
directory=/mnt
size=1G

2) ext4 on MD raid5

mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/nvme0n1 /dev/nvme1n1
mkfs.ext4 /dev/md0
mount /dev/md0 /mnt

fio script same as btrfs test

3) xfs on DM stripped target

pvcreate /dev/nvme0n1 /dev/nvme1n1
vgcreate striped_vol_group /dev/nvme0n1 /dev/nvme1n1
lvcreate -i2 -I4 -L250G -nstriped_logical_volume striped_vol_group
mkfs.xfs -f /dev/striped_vol_group/striped_logical_volume
mount /dev/striped_vol_group/striped_logical_volume /mnt

fio script same as btrfs test

------

Results:

	4.1-rc4		4.1-rc4-patched
btrfs	1818.6MB/s	1874.1MB/s
ext4	717307KB/s	714030KB/s
xfs	1396.6MB/s	1398.6MB/s