linux-kernel - [PATCH v4 1/6] docs: device-mapper: add remap_and

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Date:   Wed, 3 Feb 2021 18:53:53 +0300
From:   Sergei Shtepa <sergei.shtepa@...am.com>
To:     <Damien.LeMoal@....com>, <snitzer@...hat.com>, <hare@...e.de>,
        <ming.lei@...hat.com>, <agk@...hat.com>, <corbet@....net>,
        <axboe@...nel.dk>, <jack@...e.cz>, <johannes.thumshirn@....com>,
        <gregkh@...uxfoundation.org>, <koct9i@...il.com>, <steve@....org>,
        <dm-devel@...hat.com>, <linux-block@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <sergei.shtepa@...am.com>, <pavgel.tide@...am.com>
Subject: [PATCH v4 1/6] docs: device-mapper: add remap_and_filter

remap_and_filter - describes the new features that
blk_interposer provides for device mapper.

Signed-off-by: Sergei Shtepa <sergei.shtepa@...am.com>
---
 .../admin-guide/device-mapper/index.rst       |   1 +
 .../device-mapper/remap_and_filter.rst        | 132 ++++++++++++++++++
 2 files changed, 133 insertions(+)
 create mode 100644 Documentation/admin-guide/device-mapper/remap_and_filter.rst

diff --git a/Documentation/admin-guide/device-mapper/index.rst b/Documentation/admin-guide/device-mapper/index.rst
index 6cf8adc86fa8..e868d5bbec7e 100644
--- a/Documentation/admin-guide/device-mapper/index.rst
+++ b/Documentation/admin-guide/device-mapper/index.rst
@@ -27,6 +27,7 @@ Device Mapper
     linear
     log-writes
     persistent-data
+    remap_and_filter
     snapshot
     statistics
     striped
diff --git a/Documentation/admin-guide/device-mapper/remap_and_filter.rst b/Documentation/admin-guide/device-mapper/remap_and_filter.rst
new file mode 100644
index 000000000000..616b67998305
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/remap_and_filter.rst
@@ -0,0 +1,132 @@
+=================
+DM remap & filter
+=================
+
+Introduction
+============
+
+Usually LVM should be used for new devices.
+The administrator have to create logical volumes for the system partition
+when installing the operating system. For a running system with
+partitioned disk space and mounted file systems, it is quite difficult to
+reconfigure to logical volumes. As a result, all the features that Device
+Mapper provides are not available for non-LVM systems.
+This problem is partially solved by the dm remap functionality, which
+uses the kernel's blk_interposer.
+
+blk_interposer
+==============
+
+Blk_interposer extends the capabilities of the DM, as it allows to
+intercept and redirect bio requests for block devices that are not
+dm device. At the same time, blk_interposer allows to attach and detach
+from devices "on the fly", without stopping the execution of user
+programs.
+
+Blk_interposer allows to do two tasks: remap and filter.
+Remap allows to redirect all requests from one block device to another.
+Filter allows to do additional processing of the request, but without
+redirection. An intercepted request can get to the block device to which
+it was addressed, without changes.
+
+Remap
+=====
+
+Consider the functionality of the remap. This will allow to connect
+any block device with a dm device "on the fly".
+Suppose we have a file system mounted on the block device /dev/sda1::
+
+  +-------------+
+  | file system |
+  +-------------+
+        ||
+        \/
+  +-------------+
+  | /dev/sda1   |
+  +-------------+
+
+Creating a new DM device that will be mapped on the same /dev/sda1::
+
+  +-------------+  +-----------+
+  | file system |  | dm-linear |
+  +-------------+  +-----------+
+           ||         ||
+           \/         \/
+         +---------------+
+         |   /dev/sda1   |
+         +---------------+
+
+Redirecting all bio requests for the /dev/sda1 device to the new dm
+device::
+
+  +-------------+
+  | file system |
+  +-------------+
+        ||
+        \/
+   +----------+    +-----------+
+   |  remap   | => | dm-linear |
+   +----------+    +-----------+
+                         ||
+                         \/
+                   +-----------+
+                   | /dev/sda1 |
+                   +-----------+
+
+To achieve this, you need to:
+
+Create new dm device with option 'noexcl'. It's allow to open
+underlying block-device without the FMODE_EXCL flag::
+
+  echo "0 `blockdev --getsz $1` linear $DEV 0 noexcl" | dmsetup create dm-noexcl
+
+Call remap command::
+
+  dmsetup remap start dm-noexcl $1
+
+Remap can be used to extend the functionality of dm-snap. This will allow
+to take snapshots from any block devices, not just logical volumes.
+
+Filter
+======
+
+Filter does not redirect the bio to another device. It does not create
+a clone of the bio request. After receiving the request, the filter can
+only add some processing, complete the bio request, or return the bio
+for further processing.
+
+Suppose we have a file system mounted on the block device /dev/sda1::
+
+  +-------------+
+  | file system |
+  +-------------+
+        ||
+        \/
+  +-------------+
+  | /dev/sda1   |
+  +-------------+
+
+Creating a new dm device that will implement filter::
+
+  +-------------+
+  | file system |
+  +-------------+
+        ||
+        \/
+    +--------+    +----------+
+    | filter | => | dm-delay |
+    +--------+    +----------+
+        ||
+        \/
+  +-------------+
+  | /dev/sda1   |
+  +-------------+
+
+Using filter we can change the behavior of debugging tools:
+ * dm-dust,
+ * dm-delay,
+ * dm-flakey,
+ * dm-verity.
+
+In the new version, they are will be able to change the behavior of any
+existing block device, without creating a new one.
-- 
2.20.1