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[dm-devel] [PATCH] DM docs for Documentation/device-mapper/



Here's some additional docs for the Documentation/device-mapper/ directory.
This includes simple explinations for linear, stripe, zero, dm-io, and
kcopyd. This patch replaces the existing docs patch in the 2.6.7-rc2-udm1
(I made some changes in linear.txt and didn't feel like an incremental diff
today. :) These docs are currently aimed at how to use the relevant modules,
and not so much at how they are designed or how they work internally. I'll
get to that in the near future. I also hope to get some docs written for
snapshot and mirror soon.

-- 
Kevin Corry
kevcorry us ibm com
http://evms.sourceforge.net/



Device-Mapper documentation.

--- diff/Documentation/device-mapper/dm-io.txt	1970-01-01 00:00:00.000000000 +0000
+++ source/Documentation/device-mapper/dm-io.txt	2004-06-07 15:45:51.865318424 +0000
@@ -0,0 +1,75 @@
+dm-io
+=====
+
+Dm-io provides synchronous and asynchronous I/O services. There are three
+types of I/O services available, and each type has a sync and an async
+version.
+
+The user must set up an io_region structure to describe the desired location 
+of the I/O. Each io_region indicates a block-device along with the starting
+sector and size of the region.
+
+   struct io_region {
+      struct block_device *bdev;
+      sector_t sector;
+      sector_t count;
+   };
+
+Dm-io can read from one io_region or write to one or more io_regions. Writes
+to multiple regions are specified by an array of io_region structures.
+
+The first I/O service type takes a list of memory pages as the data buffer for
+the I/O, along with an offset into the first page.
+
+   struct page_list {
+      struct page_list *next;
+      struct page *page;
+   };
+
+   int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
+                  struct page_list *pl, unsigned int offset,
+                  unsigned long *error_bits);
+   int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
+                   struct page_list *pl, unsigned int offset,
+                   io_notify_fn fn, void *context);
+
+The second I/O service type takes an array of bio vectors as the data buffer
+for the I/O. This service can be handy if the caller has a pre-assembled bio,
+but wants to direct different portions of the bio to different devices.
+
+   int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
+                       int rw, struct bio_vec *bvec,
+                       unsigned long *error_bits);
+   int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
+                        int rw, struct bio_vec *bvec,
+                        io_notify_fn fn, void *context);
+
+The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
+data buffer for the I/O. This service can be handy if the caller needs to do
+I/O to a large region but doesn't want to allocate a large number of individual
+memory pages.
+
+   int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
+                     void *data, unsigned long *error_bits);
+   int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
+                      void *data, io_notify_fn fn, void *context);
+
+Callers of the asynchronous I/O services must include the name of a completion
+callback routine and a pointer to some context data for the I/O.
+
+   typedef void (*io_notify_fn)(unsigned long error, void *context);
+
+The "error" parameter in this callback, as well as the "*error" parameter in
+all of the synchronous versions, is a bitset (instead of a simple error value).
+In the case of an write-I/O to multiple regions, this bitset allows dm-io to
+indicate success or failure on each individual region.
+
+Before using any of the dm-io services, the user should call dm_io_get()
+and specify the number of pages they expect to perform I/O on concurrently.
+Dm-io will attempt to resize its mempool to make sure enough pages are
+always available in order to avoid unnecessary waiting while performing I/O.
+
+When the user is finished using the dm-io services, they should call
+dm_io_put() and specify the same number of pages that were given on the
+dm_io_get() call.
+
--- diff/Documentation/device-mapper/kcopyd.txt	1970-01-01 00:00:00.000000000 +0000
+++ source/Documentation/device-mapper/kcopyd.txt	2004-06-06 11:43:18.000000000 +0000
@@ -0,0 +1,47 @@
+kcopyd
+======
+
+Kcopyd provides the ability to copy a range of sectors from one block-device
+to one or more other block-devices, with an asynchronous completion
+notification. It is used by dm-snapshot and dm-mirror.
+
+Users of kcopyd must first create a client and indicate how many memory pages
+to set aside for their copy jobs. This is done with a call to
+kcopyd_client_create().
+
+   int kcopyd_client_create(unsigned int num_pages,
+                            struct kcopyd_client **result);
+
+To start a copy job, the user must set up io_region structures to describe
+the source and destinations of the copy. Each io_region indicates a
+block-device along with the starting sector and size of the region. The source
+of the copy is given as one io_region structure, and the destinations of the
+copy are given as an array of io_region structures.
+
+   struct io_region {
+      struct block_device *bdev;
+      sector_t sector;
+      sector_t count;
+   };
+
+To start the copy, the user calls kcopyd_copy(), passing in the client
+pointer, pointers to the source and destination io_regions, the name of a
+completion callback routine, and a pointer to some context data for the copy.
+
+   int kcopyd_copy(struct kcopyd_client *kc, struct io_region *from,
+                   unsigned int num_dests, struct io_region *dests,
+                   unsigned int flags, kcopyd_notify_fn fn, void *context);
+
+   typedef void (*kcopyd_notify_fn)(int read_err, unsigned int write_err,
+				    void *context);
+
+When the copy completes, kcopyd will call the user's completion routine,
+passing back the user's context pointer. It will also indicate if a read or
+write error occurred during the copy.
+
+When a user is done with all their copy jobs, they should call
+kcopyd_client_destroy() to delete the kcopyd client, which will release the
+associated memory pages.
+
+   void kcopyd_client_destroy(struct kcopyd_client *kc);
+
--- diff/Documentation/device-mapper/linear.txt	1970-01-01 00:00:00.000000000 +0000
+++ source/Documentation/device-mapper/linear.txt	2004-06-06 20:29:48.000000000 +0000
@@ -0,0 +1,61 @@
+dm-linear
+=========
+
+Device-Mapper's "linear" target maps a linear range of the Device-Mapper
+device onto a linear range of another device.  This is the basic building
+block of logical volume managers.
+
+Parameters: <dev path> <offset>
+    <dev path>: Full pathname to the underlying block-device, or a
+                "major:minor" device-number.
+    <offset>: Starting LBA within the device.
+
+
+Example scripts
+===============
+[[
+#!/bin/sh
+# Create an identity mapping for a device
+echo "0 `blockdev --getsize $1` linear $1 0" | dmsetup create identity
+]]
+
+
+[[
+#!/bin/sh
+# Join 2 devices together
+size1=`blockdev --getsize $1`
+size2=`blockdev --getsize $2`
+echo "0 $size1 linear $1 0
+$size1 $size2 linear $2 0" | dmsetup create joined
+]]
+
+
+[[
+#!/usr/bin/perl -w
+# Split a device into 4M chunks and then join them together in reverse order.
+
+my $name = "reverse";
+my $extent_size = 4 * 1024 * 2;
+my $dev = $ARGV[0];
+my $table = "";
+my $count = 0;
+
+if (!defined($dev)) {
+        die("Please specify a device.\n");
+}
+
+my $dev_size = `blockdev --getsize $dev`;
+my $extents = int($dev_size / $extent_size) -
+              (($dev_size % $extent_size) ? 1 : 0);
+
+while ($extents > 0) {
+        my $this_start = $count * $extent_size;
+        $extents--;
+        $count++;
+        my $this_offset = $extents * $extent_size;
+
+        $table .= "$this_start $extent_size linear $dev $this_offset\n";
+}
+
+`echo \"$table\" | dmsetup create $name`;
+]]
--- diff/Documentation/device-mapper/striped.txt	1970-01-01 00:00:00.000000000 +0000
+++ source/Documentation/device-mapper/striped.txt	2004-06-06 12:24:56.000000000 +0000
@@ -0,0 +1,57 @@
+dm-stripe
+=========
+
+Device-Mapper's "striped" target is used to create a striped (i.e. RAID-0)
+device across one or more underlying devices. Data is written in "chunks",
+with consecutive chunks rotating among the underlying devices. This can
+potentially provide improved I/O throughput by utilizing several physical
+devices in parallel.
+
+Parameters: <num devs> <chunk size> [<dev path> <offset>]+
+    <num devs>: Number of underlying devices.
+    <chunk size>: Size of each chunk of data. Must be a power-of-2.
+    <dev path>: Full pathname to the underlying block-device, or a
+                "major:minor" device-number.
+    <offset>: Starting LBA within the device.
+
+One or more underlying devices can be specified. The striped device size must
+be a multiple of the chunk size and a multiple of the number of underlying
+devices.
+
+
+Example scripts
+===============
+
+[[
+#!/usr/bin/perl -w
+# Create a striped device across any number of underlying devices. The device
+# will be called "stripe_dev" and have a chunk-size of 128k.
+
+my $chunk_size = 128 * 2;
+my $dev_name = "stripe_dev";
+my $num_devs = @ARGV;
+my @devs = @ARGV;
+my ($min_dev_size, $stripe_dev_size, $i);
+
+if (!$num_devs) {
+        die("Specify at least one device\n");
+}
+
+$min_dev_size = `blockdev --getsize $devs[0]`;
+for ($i = 1; $i < $num_devs; $i++) {
+        my $this_size = `blockdev --getsize $devs[$i]`;
+        $min_dev_size = ($min_dev_size < $this_size) ?
+                        $min_dev_size : $this_size;
+}
+
+$stripe_dev_size = $min_dev_size * $num_devs;
+$stripe_dev_size -= $stripe_dev_size % ($chunk_size * $num_devs);
+
+$table = "0 $stripe_dev_size striped $num_devs $chunk_size";
+for ($i = 0; $i < $num_devs; $i++) {
+        $table .= " $devs[$i] 0";
+}
+
+`echo $table | dmsetup create $dev_name`;
+]]
+
--- diff/Documentation/device-mapper/zero.txt	1970-01-01 00:00:00.000000000 +0000
+++ source/Documentation/device-mapper/zero.txt	2004-06-07 16:25:23.066841000 +0000
@@ -0,0 +1,37 @@
+dm-zero
+=======
+
+Device-Mapper's "zero" target provides a block-device that always returns
+zero'd data on reads and silently drops writes. This is similar behavior to
+/dev/zero, but as a block-device instead of a character-device.
+
+Dm-zero has no target-specific parameters.
+
+One very interesting use of dm-zero is for creating "sparse" devices in
+conjunction with dm-snapshot. A sparse device reports a device-size larger
+than the amount of actual storage space available for that device. A user can
+write data anywhere within the sparse device and read it back like a normal
+device. Reads to previously unwritten areas will return a zero'd buffer. When
+enough data has been written to fill up the actual storage space, the sparse
+device is deactivated. This can be very useful for testing device and
+filesystem limitations.
+
+To create a sparse device, start by creating a dm-zero device that's the
+desired size of the sparse device. For this example, we'll assume a 10TB
+sparse device.
+
+TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2`   # 10 TB in sectors
+echo "0 $TEN_TERABYTES zero" | dmsetup create zero1
+
+Then create a snapshot of the zero device, using any available block-device as
+the COW device. The size of the COW device will determine the amount of real
+space available to the sparse device. For this example, we'll assume /dev/sdb1
+is an available 10GB partition.
+
+echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
+   dmsetup create sparse1
+
+This will create a 10TB sparse device called /dev/mapper/sparse1 that has
+10GB of actual storage space available. If more than 10GB of data is written
+to this device, it will start returning I/O errors.
+

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