[PATCH v4 0/2] dm-unstripe

[PATCH v4 0/2] dm-unstripe

[Scott Bauer]

Change log:

v3->v4:
 Addressed comments from Randy. Modified documentation to be clearer,
 and fixed small off by one in bounds checking in constructor.

v2->v3:

1) Renamed variables in-code to reflect correct terminology with respect
   to the dm-stripe target.

2) Fixed a  __must_check missing check in the constructor.

3) Used correct types for working with sector remaping.

3) Fixed documentation to reflect the correct termonology, like the code.
   Added the test script Keith sent out in an email that makes a striped
   device and uses the un-stripe target to access the underlying loop
   devices.

[PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Scott Bauer]

This device mapper module remaps and unstripes IO so it lands
solely on a single drive in a RAID 0/dm-stripe target.
In a 4 drive RAID 0 the mapper exposes 1/4th of the LBA range
as a virtual drive. Each IO to that virtual drive will land on
only one of the 4 drives, selected by the user.

Signed-off-by: Scott Bauer <scott.bauer@intel.com>
Acked-by: Keith Busch <keith.busch@intel.com>
---
 drivers/md/Kconfig       |  10 +++
 drivers/md/Makefile      |   1 +
 drivers/md/dm-unstripe.c | 204 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 215 insertions(+)
 create mode 100644 drivers/md/dm-unstripe.c

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 83b9362be09c..e1c48a7f98f1 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -269,6 +269,16 @@ config DM_BIO_PRISON
 
 source "drivers/md/persistent-data/Kconfig"
 
+config DM_UN_STRIPE
+       tristate "Transpose IO to individual drives on a raid device"
+       depends on BLK_DEV_DM
+       ---help---
+	  Enable this feature if you with to unstripe I/O on a RAID 0
+	  device to the respective drive. If your hardware has physical
+	  RAID 0 this module can unstripe the I/O to respective sides.
+
+	  If unsure say N.
+
 config DM_CRYPT
 	tristate "Crypt target support"
 	depends on BLK_DEV_DM
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index f701bb211783..2cc380b71319 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -43,6 +43,7 @@ obj-$(CONFIG_BCACHE)		+= bcache/
 obj-$(CONFIG_BLK_DEV_MD)	+= md-mod.o
 obj-$(CONFIG_BLK_DEV_DM)	+= dm-mod.o
 obj-$(CONFIG_BLK_DEV_DM_BUILTIN) += dm-builtin.o
+obj-$(CONFIG_DM_UN_STRIPE)   += dm-unstripe.o
 obj-$(CONFIG_DM_BUFIO)		+= dm-bufio.o
 obj-$(CONFIG_DM_BIO_PRISON)	+= dm-bio-prison.o
 obj-$(CONFIG_DM_CRYPT)		+= dm-crypt.o
diff --git a/drivers/md/dm-unstripe.c b/drivers/md/dm-unstripe.c
new file mode 100644
index 000000000000..086689fb11e3
--- /dev/null
+++ b/drivers/md/dm-unstripe.c
@@ -0,0 +1,204 @@
+/*
+ * Copyright © 2017 Intel Corporation
+ *
+ * Authors:
+ *    Scott  Bauer      <scott.bauer@intel.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ */
+
+#include "dm.h"
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/blkdev.h>
+#include <linux/bio.h>
+#include <linux/slab.h>
+#include <linux/bitops.h>
+#include <linux/device-mapper.h>
+
+
+struct unstripe {
+	struct dm_dev *ddisk;
+	sector_t chunk_sectors;
+	sector_t stripe_sectors;
+	u8 chunk_shift;
+	u8 cur_drive;
+};
+
+
+#define DM_MSG_PREFIX "dm-unstripe"
+static const char *parse_err = "Please provide the necessary information:"
+	"<drive> <device (0 indexed)> <total_devices>"
+	" <chunk size in 512B sectors || 0 to use max hw sector size>";
+
+/*
+ * Argument layout:
+ * <drive> <stripe/drive to extract (0 indexed)>
+ *         <total_devices> <chunk size in 512B sect>
+ */
+static int set_ctr(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct block_device *bbdev;
+	struct unstripe *target;
+	unsigned int chunk_size;
+	u64 tot_sec, mod;
+	u8 cur_drive, tot_drives;
+	char dummy;
+	int ret;
+
+	if (argc != 4) {
+		DMERR("%s", parse_err);
+		return -EINVAL;
+	}
+
+	if (sscanf(argv[1], "%hhu%c", &cur_drive, &dummy) != 1 ||
+	    sscanf(argv[2], "%hhu%c", &tot_drives, &dummy) != 1 ||
+	    sscanf(argv[3], "%u%c", &chunk_size, &dummy) != 1) {
+		DMERR("%s", parse_err);
+		return -EINVAL;
+	}
+
+	if (tot_drives == 0 || (cur_drive >= tot_drives && tot_drives > 1)) {
+		DMERR("Please provide a drive between [0,%hhu)", tot_drives);
+		return -EINVAL;
+	}
+
+	target = kzalloc(sizeof(*target), GFP_KERNEL);
+
+	if (!target) {
+		DMERR("Failed to allocate space for DM unstripe!");
+		return -ENOMEM;
+	}
+
+	ret = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table),
+			    &target->ddisk);
+	if (ret) {
+		kfree(target);
+		DMERR("dm-unstripe dev lookup failure! for drive %s", argv[0]);
+		return ret;
+	}
+
+	bbdev = target->ddisk->bdev;
+
+	target->cur_drive = cur_drive;
+	if (chunk_size)
+		target->chunk_sectors = chunk_size;
+	else
+		target->chunk_sectors =
+			queue_max_hw_sectors(bdev_get_queue(bbdev));
+
+	target->stripe_sectors = (tot_drives - 1) * target->chunk_sectors;
+	target->chunk_shift = fls(target->chunk_sectors) - 1;
+
+	ret = dm_set_target_max_io_len(ti, target->chunk_sectors);
+	if (ret) {
+		dm_put_device(ti, target->ddisk);
+		kfree(target);
+		DMERR("Failed to set max io len!");
+		return ret;
+	}
+	ti->private = target;
+
+	tot_sec = i_size_read(bbdev->bd_inode) >> SECTOR_SHIFT;
+	mod = tot_sec % target->chunk_sectors;
+
+	if (ti->len == 1)
+		ti->len = (tot_sec / tot_drives) - mod;
+	ti->begin = 0;
+	return 0;
+}
+
+static void set_dtr(struct dm_target *ti)
+{
+	struct unstripe *target = ti->private;
+
+	dm_put_device(ti, target->ddisk);
+	kfree(target);
+}
+
+
+static sector_t map_to_core(struct dm_target *ti, struct bio *bio)
+{
+	struct unstripe *target = ti->private;
+	unsigned long long sec = bio->bi_iter.bi_sector;
+	unsigned long long group;
+
+	group = (sec >> target->chunk_shift);
+	/* Account for what drive we're operating on */
+	sec += (target->cur_drive * target->chunk_sectors);
+	/* Shift us up to the right "row" on the drive*/
+	sec += target->stripe_sectors * group;
+	return sec;
+}
+
+static int set_map_bio(struct dm_target *ti, struct bio *bio)
+{
+	struct unstripe *target = ti->private;
+
+	if (bio_sectors(bio))
+		bio->bi_iter.bi_sector = map_to_core(ti, bio);
+
+	bio_set_dev(bio, target->ddisk->bdev);
+	submit_bio(bio);
+	return DM_MAPIO_SUBMITTED;
+}
+
+static void set_iohints(struct dm_target *ti,
+			struct queue_limits *limits)
+{
+	struct unstripe *target = ti->private;
+	struct queue_limits *lim = &bdev_get_queue(target->ddisk->bdev)->limits;
+
+	blk_limits_io_min(limits, lim->io_min);
+	blk_limits_io_opt(limits, lim->io_opt);
+	limits->chunk_sectors = target->chunk_sectors;
+}
+
+static int set_iterate(struct dm_target *ti, iterate_devices_callout_fn fn,
+		       void *data)
+{
+	struct unstripe *target = ti->private;
+
+	return fn(ti, target->ddisk, 0, ti->len, data);
+}
+
+static struct target_type iset_target = {
+	.name = "unstripe",
+	.version = {1, 0, 0},
+	.module = THIS_MODULE,
+	.ctr = set_ctr,
+	.dtr = set_dtr,
+	.map = set_map_bio,
+	.iterate_devices = set_iterate,
+	.io_hints = set_iohints,
+};
+
+static int __init dm_unstripe_init(void)
+{
+	int r = dm_register_target(&iset_target);
+
+	if (r < 0)
+		DMERR("register failed %d", r);
+
+	return r;
+}
+
+static void __exit dm_unstripe_exit(void)
+{
+	dm_unregister_target(&iset_target);
+}
+
+module_init(dm_unstripe_init);
+module_exit(dm_unstripe_exit);
+
+MODULE_DESCRIPTION(DM_NAME " DM unstripe");
+MODULE_ALIAS("dm-unstripe");
+MODULE_AUTHOR("Scott Bauer <scott.bauer@intel.com>");
+MODULE_LICENSE("GPL");
-- 
2.11.0

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Randy Dunlap]

On 12/18/2017 09:28 AM, Scott Bauer wrote:
> This device mapper module remaps and unstripes IO so it lands
> solely on a single drive in a RAID 0/dm-stripe target.
> In a 4 drive RAID 0 the mapper exposes 1/4th of the LBA range
> as a virtual drive. Each IO to that virtual drive will land on
> only one of the 4 drives, selected by the user.
> 
> Signed-off-by: Scott Bauer <scott.bauer@intel.com>
> Acked-by: Keith Busch <keith.busch@intel.com>
> ---
>  drivers/md/Kconfig       |  10 +++
>  drivers/md/Makefile      |   1 +
>  drivers/md/dm-unstripe.c | 204 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 215 insertions(+)
>  create mode 100644 drivers/md/dm-unstripe.c
> 
> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> index 83b9362be09c..e1c48a7f98f1 100644
> --- a/drivers/md/Kconfig
> +++ b/drivers/md/Kconfig
> @@ -269,6 +269,16 @@ config DM_BIO_PRISON
>  
>  source "drivers/md/persistent-data/Kconfig"
>  
> +config DM_UN_STRIPE
> +       tristate "Transpose IO to individual drives on a raid device"
> +       depends on BLK_DEV_DM
> +       ---help---
> +	  Enable this feature if you with to unstripe I/O on a RAID 0
> +	  device to the respective drive. If your hardware has physical
> +	  RAID 0 this module can unstripe the I/O to respective sides.

What does "sides" mean above?

> +
> +	  If unsure say N.
> +
thanks,
-- 
~Randy

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Scott Bauer]

> > +config DM_UN_STRIPE
> > +       tristate "Transpose IO to individual drives on a raid device"
> > +       depends on BLK_DEV_DM
> > +       ---help---
> > +	  Enable this feature if you with to unstripe I/O on a RAID 0
> > +	  device to the respective drive. If your hardware has physical
> > +	  RAID 0 this module can unstripe the I/O to respective sides.
> 
> What does "sides" mean above?

Should say, "members". There's also an alternative patch set out by Heinz
that I am currently testing/writing comments for. It looks like we'll use
his version after I submit my comments (IE take the stuff I put in my v4
to fix your comments).

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Mike Snitzer]

On Mon, Dec 18 2017 at  1:52pm -0500,
Scott Bauer <scott.bauer@intel.com> wrote:

> 
> > > +config DM_UN_STRIPE
> > > +       tristate "Transpose IO to individual drives on a raid device"
> > > +       depends on BLK_DEV_DM
> > > +       ---help---
> > > +	  Enable this feature if you with to unstripe I/O on a RAID 0
> > > +	  device to the respective drive. If your hardware has physical
> > > +	  RAID 0 this module can unstripe the I/O to respective sides.
> > 
> > What does "sides" mean above?
> 
> Should say, "members". There's also an alternative patch set out by Heinz
> that I am currently testing/writing comments for. It looks like we'll use
> his version after I submit my comments (IE take the stuff I put in my v4
> to fix your comments).

Can you be more specific?  I've not seen anything from Heinz.

I've been reviewing your v4 and modified it to match dm-stripe.c more
(like Alasdair asked for).

Mike

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Mike Snitzer]

On Mon, Dec 18 2017 at  2:32pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Mon, Dec 18 2017 at  1:52pm -0500,
> Scott Bauer <scott.bauer@intel.com> wrote:
> 
> > 
> > > > +config DM_UN_STRIPE
> > > > +       tristate "Transpose IO to individual drives on a raid device"
> > > > +       depends on BLK_DEV_DM
> > > > +       ---help---
> > > > +	  Enable this feature if you with to unstripe I/O on a RAID 0
> > > > +	  device to the respective drive. If your hardware has physical
> > > > +	  RAID 0 this module can unstripe the I/O to respective sides.
> > > 
> > > What does "sides" mean above?
> > 
> > Should say, "members". There's also an alternative patch set out by Heinz
> > that I am currently testing/writing comments for. It looks like we'll use
> > his version after I submit my comments (IE take the stuff I put in my v4
> > to fix your comments).
> 
> Can you be more specific?  I've not seen anything from Heinz.
> 
> I've been reviewing your v4 and modified it to match dm-stripe.c more
> (like Alasdair asked for).

When the right hand isn't aware the left is doing something ;)

I'll combine my effort with Heinz's.

Please don't send another revision until you see what I've come up with.

Thanks,
Mike

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[kbuild test robot]

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Hi Scott,

I love your patch! Yet something to improve:

[auto build test ERROR on dm/for-next]
[also build test ERROR on v4.15-rc4]
[cannot apply to next-20171220]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Scott-Bauer/dm-unstripe/20171220-121845
base:   https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git for-next
config: m68k-allyesconfig (attached as .config)
compiler: m68k-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=m68k 

All errors (new ones prefixed by >>):

   drivers/md/dm-unstripe.o: In function `set_ctr':
>> dm-unstripe.c:(.text+0x36a): undefined reference to `__udivdi3'
>> dm-unstripe.c:(.text+0x384): undefined reference to `__umoddi3'

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
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Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 1186 bytes --]

Hi Scott,

I love your patch! Yet something to improve:

[auto build test ERROR on dm/for-next]
[also build test ERROR on v4.15-rc4]
[cannot apply to next-20171220]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Scott-Bauer/dm-unstripe/20171220-121845
base:   https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git for-next
config: arm-allyesconfig (attached as .config)
compiler: arm-linux-gnueabi-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=arm 

All errors (new ones prefixed by >>):

   drivers/md/dm-unstripe.o: In function `set_ctr':
>> dm-unstripe.c:(.text+0x438): undefined reference to `__aeabi_uldivmod'
   dm-unstripe.c:(.text+0x450): undefined reference to `__aeabi_uldivmod'

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 63931 bytes --]

Re: [PATCH v4 1/2] dm-unstripe: unstripe RAID 0/dm-striped device

[Mike Snitzer]

On Wed, Dec 20 2017 at  5:21am -0500,
kbuild test robot <lkp@intel.com> wrote:

> Hi Scott,
> 
> I love your patch! Yet something to improve:
> 
> [auto build test ERROR on dm/for-next]
> [also build test ERROR on v4.15-rc4]
> [cannot apply to next-20171220]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
> 
> url:    https://github.com/0day-ci/linux/commits/Scott-Bauer/dm-unstripe/20171220-121845
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git for-next
> config: arm-allyesconfig (attached as .config)
> compiler: arm-linux-gnueabi-gcc (Debian 7.2.0-11) 7.2.0
> reproduce:
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # save the attached .config to linux build tree
>         make.cross ARCH=arm 
> 
> All errors (new ones prefixed by >>):
> 
>    drivers/md/dm-unstripe.o: In function `set_ctr':
> >> dm-unstripe.c:(.text+0x438): undefined reference to `__aeabi_uldivmod'
>    dm-unstripe.c:(.text+0x450): undefined reference to `__aeabi_uldivmod'
> 
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation


These reports are no longer relevant given the code has changed since
v4. In particular these 64-bit divide and mod in v4 no longer exist:

+       u64 tot_sec, mod;
...
+       tot_sec = i_size_read(bbdev->bd_inode) >> SECTOR_SHIFT;
+       mod = tot_sec % target->chunk_sectors;
+
+       if (ti->len == 1)
+               ti->len = (tot_sec / tot_drives) - mod;

[PATCH v4 2/2] dm-unstripe: Add documentation for unstripe target

[Scott Bauer]

Signed-off-by: Scott Bauer <scott.bauer@intel.com>
---
 Documentation/device-mapper/dm-unstripe.txt | 130 ++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)
 create mode 100644 Documentation/device-mapper/dm-unstripe.txt

diff --git a/Documentation/device-mapper/dm-unstripe.txt b/Documentation/device-mapper/dm-unstripe.txt
new file mode 100644
index 000000000000..9c13e9316d52
--- /dev/null
+++ b/Documentation/device-mapper/dm-unstripe.txt
@@ -0,0 +1,130 @@
+Device-Mapper Unstripe
+=====================
+
+The device-mapper unstripe (dm-unstripe) target provides a transparent
+mechanism to unstripe a device-mapper "striped" target to access the
+underlying disks without having to touch the true backing block-device.
+It can also be used to unstripe a hardware RAID-0 to access backing disks
+as well.
+
+
+Parameters:
+<drive (ex: /dev/nvme0n1)> <drive #> <# of drives> <chunk sectors>
+
+
+<drive>
+	The block device you wish to unstripe.
+
+<drive #>
+        The physical drive you wish to expose via this "virtual" device
+	mapper target. This must be 0 indexed.
+
+<# of drives>
+        The number of drives in the RAID 0.
+
+<chunk sectors>
+	The amount of 512B sectors in the chunk striping, or zero, if you
+	wish you use max_hw_sector_size.
+
+
+Why use this module?
+=====================
+
+    An example of undoing an existing dm-stripe:
+
+    This small bash script will setup 4 loop devices and use the existing
+    dm-stripe target to combine the 4 devices into one. It then will use
+    the unstripe target on the new combined stripe device to access the
+    individual backing loop devices. We write data to the newly exposed
+    unstriped devices and verify the data written matches the correct
+    underlying device on the striped array.
+
+    #!/bin/bash
+
+    MEMBER_SIZE=$((128 * 1024 * 1024))
+    NUM=4
+    SEQ_END=$((${NUM}-1))
+    CHUNK=256
+    BS=4096
+
+    RAID_SIZE=$((${MEMBER_SIZE}*${NUM}/512))
+    DM_PARMS="0 ${RAID_SIZE} striped ${NUM} ${CHUNK}"
+    COUNT=$((${MEMBER_SIZE} / ${BS}))
+
+    for i in $(seq 0 ${SEQ_END}); do
+      dd if=/dev/zero of=member-${i} bs=${MEMBER_SIZE} count=1 oflag=direct
+      losetup /dev/loop${i} member-${i}
+      DM_PARMS+=" /dev/loop${i} 0"
+    done
+
+    echo $DM_PARMS | dmsetup create raid0
+    for i in $(seq 0 ${SEQ_END}); do
+      echo "0 1 unstripe /dev/mapper/raid0 ${i} ${NUM} ${CHUNK}" | dmsetup create set-${i}
+    done;
+
+    for i in $(seq 0 ${SEQ_END}); do
+      dd if=/dev/urandom of=/dev/mapper/set-${i} bs=${BS} count=${COUNT} oflag=direct
+      diff /dev/mapper/set-${i} member-${i}
+    done;
+
+    for i in $(seq 0 ${SEQ_END}); do
+      dmsetup remove set-${i}
+    done
+
+    dmsetup remove raid0
+
+    for i in $(seq 0 ${SEQ_END}); do
+      losetup -d /dev/loop${i}
+      rm -f member-${i}
+    done
+
+==============
+
+
+    Another example:
+
+    Intel NVMe drives contain two cores on the physical device.
+    Each core of the drive has segregated access to its LBA range.
+    The current LBA model has a RAID 0 128k chunk on each core, resulting
+    in a 256k stripe across the two cores:
+
+       Core 0:                Core 1:
+      __________            __________
+      | LBA 512|            | LBA 768|
+      | LBA 0  |            | LBA 256|
+      ----------	    ----------
+
+    The purpose of this unstriping is to provide better QoS in noisy
+    neighbor environments. When two partitions are created on the
+    aggregate drive without this unstriping, reads on one partition
+    can affect writes on another partition. This is because the partitions
+    are striped across the two cores. When we unstripe this hardware RAID 0
+    and make partitions on each new exposed device the two partitions are now
+    physically separated.
+
+    With the module we were able to segregate a fio script that has read and
+    write jobs that are independent of each other. Compared to when we run
+    the test on a combined drive with partitions, we were able to get a 92%
+    reduction read latency using this device mapper target.
+
+
+====================
+Example scripts:
+
+
+dmsetup create nvmset1 --table '0 1 unstripe /dev/nvme0n1 1 2 0'
+dmsetup create nvmset0 --table '0 1 unstripe /dev/nvme0n1 0 2 0'
+
+There will now be two mappers:
+/dev/mapper/nvmset1
+/dev/mapper/nvmset0
+
+that will expose core 0 and core 1.
+
+
+# In a dm-stripe with 4 drives of chunk size 128K:
+dmsetup create raid_disk0 --table '0 1 unstripe /dev/mapper/striped 0 4 256'
+dmsetup create raid_disk1 --table '0 1 unstripe /dev/mapper/striped 1 4 256'
+dmsetup create raid_disk2 --table '0 1 unstripe /dev/mapper/striped 2 4 256'
+dmsetup create raid_disk3 --table '0 1 unstripe /dev/mapper/striped 3 4 256'
+
-- 
2.11.0