Re: [PATCH 0/2] dm: add new loop and ram targets

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jan 17 2018 at  2:33pm -0500,
Heinz Mauelshagen <heinzm@redhat.com> wrote:

> Enhancing IO performance compared to the kernels existing
> loop driver thus better suiting respective requirements in
> test setups, this patch series adds new "loop" and "ram" targets.
> 
> For measures see test results below.
> 
> 
> The "loop" target maps segments to backing files.
> Mapping table example:
> 0 4192256 loop /tmp/mapper_loop1
> 4192256 2097152 loop /dev/nvm/mapper_loop0
> 
> 
> The "ram" target accesses RAM directly rather than through
> tmpfs additionally enhancing performance compared to "loop"
> thus avoding filesystem overhead.
> Mapping table example:
> 0 8388608 ram
> 
> "ram" is a singleton target.
> 
> 
> Performance test results for 4K and 32K IOPS comparing the loop driver
> with dm-loop backed by tmpfs and dm-ram (all 2GiB backing size):
> 
> <TESTSCRIPT>
> #!/bin/sh
> for f in /tmp/loop0 /tmp/mapper_loop0
> do
> 	dd if=/dev/zero of=$f bs=256M count=8 iflag=fullblock
> done
> 
> losetup /dev/loop0 /tmp/loop0
> sectors=`du -s /tmp/mapper_loop0|cut -f1`
> dmsetup create loop0 --table "0 $sectors loop /tmp/mapper_loop0"
> dmsetup create ram --table "0 $sectors ram"
> 
> for bs in 4K 32K
> do
> 	for d in /dev/loop0 /dev/mapper/loop0 /dev/mapper/ram
> 	do
> 		echo 3 > /proc/sys/vm/drop_caches
> 		fio --bs=$bs --rw=randrw --numjobs=99 --group_reporting --iodepth=12 --runtime=3 --ioengine=libaio \
> 		    --loops=1 --direct=1 --exitall --name dc --filename=$d | egrep "read|write"
> 	done
> done
> </TESTSCRIPT>
> 
> <4K_RESULTS>
> loop driver:
>    read: IOPS=226k, BW=881MiB/s (924MB/s)(2645MiB/3003msec)
>   write: IOPS=225k, BW=880MiB/s (923MB/s)(2643MiB/3003msec)
> dm-loop target:
>    read: IOPS=425k, BW=1661MiB/s (1742MB/s)(4990MiB/3004msec)
>   write: IOPS=425k, BW=1662MiB/s (1743MB/s)(4992MiB/3004msec)
> dm-ram target:
>    read: IOPS=636k, BW=2484MiB/s (2605MB/s)(7464MiB/3005msec)
>   write: IOPS=636k, BW=2484MiB/s (2605MB/s)(7464MiB/3005msec)
> </4K_RESULTS>
> 
> <32K_RESULTS>
> loop driver:
>   read: IOPS=55.5k, BW=1733MiB/s (1817MB/s)(5215MiB/3009msec)
>   write: IOPS=55.2k, BW=1726MiB/s (1810MB/s)(5195MiB/3009msec)
> dm-loop target:
>    read: IOPS=110k, BW=3452MiB/s (3620MB/s)(10.1GiB/3006msec)
>   write: IOPS=110k, BW=3448MiB/s (3615MB/s)(10.1GiB/3006msec)
> dm-ram target:
>    read: IOPS=355k, BW=10.8GiB/s (11.6GB/s)(32.6GiB/3008msec)
>   write: IOPS=355k, BW=10.8GiB/s (11.6GB/s)(32.6GiB/3008msec)
> </32K_RESULTS>
> 
> 
> Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
> 
> Heinz Mauelshagen (2):
>   dm loop: new target redirecting io to backing file(s)
>   dm ram: new target redirecting io to RAM
> 
>  Documentation/device-mapper/loop.txt |  20 ++
>  Documentation/device-mapper/ram.txt  |  15 ++
>  drivers/md/Kconfig                   |  14 ++
>  drivers/md/Makefile                  |   2 +
>  drivers/md/dm-loop.c                 | 352 +++++++++++++++++++++++++++++++++++
>  drivers/md/dm-ram.c                  | 101 ++++++++++
>  6 files changed, 504 insertions(+)
>  create mode 100644 Documentation/device-mapper/loop.txt
>  create mode 100644 Documentation/device-mapper/ram.txt
>  create mode 100644 drivers/md/dm-loop.c
>  create mode 100644 drivers/md/dm-ram.c

My initial thought for dm-ram was: why?  (considering we have brd and
pmem and null_blk).  But for 100 lines of code if nothing else it could
serve as yet another example DM target for those interested in learning
more about how to implement a DM target.  Would be good to compare its
performance with brd, null_blk and pmem though.

As for dm-loop, doubling the performance of the loopback driver is quite
nice (especially with only 1/7 the number of lines of code as
drives/block/loop.c).

I'll review both of these closer in the coming days but it is getting
_really_ close to the 4.16 merge window (likley opens Sunday)  So they
may have to wait until 4.17.  We'll see.

Thanks,
Mike