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Date: Mon, 11 Dec 2017 09:00:19 -0700
From: Scott Bauer <scott.bauer@...el.com>
To: dm-devel@...hat.com
Cc: snitzer@...hat.com, agk@...hat.com, linux-kernel@...r.kernel.org,
keith.busch@...el.com, jonathan.derrick@...el.com,
Scott Bauer <scott.bauer@...el.com>
Subject: [PATCH v2 2/2] dm unstripe: Add documentation for unstripe target
Signed-off-by: Scott Bauer <scott.bauer@...el.com>
---
Documentation/device-mapper/dm-unstripe.txt | 82 +++++++++++++++++++++++++++++
1 file changed, 82 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..4e1a0a39a689
--- /dev/null
+++ b/Documentation/device-mapper/dm-unstripe.txt
@@ -0,0 +1,82 @@
+Device-Mapper Unstripe
+=====================
+
+The device-mapper Unstripe (dm-unstripe) target provides a transparent
+mechanism to unstripe a RAID 0 striping to access segregated disks.
+
+This module should be used by users who understand what the underlying
+disks look like behind the software/hardware RAID.
+
+Parameters:
+<drive (ex: /dev/nvme0n1)> <drive #> <# of drives> <stripe 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.
+
+<stripe sectors>
+ The amount of 512B sectors in the raid striping, or zero, if you
+ wish you use max_hw_sector_size.
+
+
+Why use this module?
+=====================
+
+As a use case:
+
+
+ As an 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 stripe across the two cores:
+
+ Core 0: Core 1:
+ __________ __________
+ | LBA 511| | 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. With the striping concurrent
+ reads and writes and I/O on opposite cores have lower completion times,
+ and better tail latencies.
+
+ 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 in five-9ths read latency using this device mapper target.
+
+
+ One could use the module to Logical de-pop a HDD if you have sufficient
+ geometry information regarding the drive.
+
+
+Example scripts:
+====================
+
+dmsetup create nvmset1 --table '0 1 dm-unstripe /dev/nvme0n1 1 2 0'
+dmsetup create nvmset0 --table '0 1 dm-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 Raid 0 with 4 drives of stripe size 128K:
+dmsetup create raid_disk0 --table '0 1 dm-unstripe /dev/nvme0n1 0 4 256'
+dmsetup create raid_disk1 --table '0 1 dm-unstripe /dev/nvme0n1 1 4 256'
+dmsetup create raid_disk2 --table '0 1 dm-unstripe /dev/nvme0n1 2 4 256'
+dmsetup create raid_disk3 --table '0 1 dm-unstripe /dev/nvme0n1 3 4 256'
+
--
2.11.0
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