The concept of device mapper target

Device mapper layer - explored

Every storage system requires viewing the real disks in their own way. For example, if there are four disk and the system wants to create mirroring over them or concatenate them viewing them as a single big device or if there is one disk, then a particular system may want to encrypt all the data that is going to be stored on that disk i.e. all such systems want a logical layer of block devices on the top of existing underlying block devices and map the requests on this logical layer to underlying layer after doing particular processing at this layer. This facility is provided by device mapper.Device mapper can be defined as a generic way to add required functionality in the storage stack by creating virtual layer of block devices and mapping them to existing block devices. It create virtual layers of block devices that can do different things on top of existing underlying block devices like striping, concatenation, mirroring, snapshot, etc.The Device Mapper is a modular kernel driver that provides a generic framework for volume management. It has been introduced in the kernel version 2.6. The device-mapper is used by the LVM2 and EVMS 2.x tools.LVM is a tool which allows creating and managing partitions. The first version of LVM is in kernel 2.4 where there is no concept of device mapper. So, all the management of logical layer was the responsibility of LVM. But, due to the concept of device mapper in kernel 2.6, the logical layer management is done by device mapper and the code of LVM2 has been simplified to a great extent. This also brings modularity and reusability in the kernel.

The concept of device mapper target:

As stated above, we can create various logical layers through device mapper to carry out the required functionality. Each such layer is created by defining “a device mapper target” for that layer.There is one to one correspondence between a virtual layer at device mapper layer and the dm target for that layer. The particular dm target contains the code which does the task of  implementing functionality which the virtual layer intends to do. For example, a device mapper target can be written to implement mirroring over existing block devices. This dm target shows a virtual layer to upper layers which does the task of mirroring.Currently seven such features have been added to device mapper through seven device mapper targets. The device mapper targets are as follows:

 Linear
This dm target allows us to concatenate number of disks to view them as a single big device or to view a part of disk as an individual logical disk. Thus, it creates a linear logical device on the top of existing block devices.

 RAID-0 / Striped
The striped dm target is designed to handle striping across physical volumes i.e. to carry out well known RAID-0 functionality.

 RAID-1 / Mirrored RAID
The mirrored dm target is designed to handle mirroring across number of disks. It carries out one of most famous RAID level functionality by creating and maintaining number of mirrors of data which all contain same data to improve reliability and speed of operations through load balancing.

 Snapshot
This dm target carries out the functionality of snapshot and allows accessing the older versions of all files along with the latest one.

 DM-Crypt
The dm-crypt device mapper target does the task of providing security through encrypting and decrypting of all the data that gets stored on the disk.

 Multipath  
In order to provide higher reliability for accessing the disks, this dm target provides a functionality of multipath so that in case of failure of path to disk, data on the disk can be accessed through alternative path.

 Zero
The zero dm target returns all the data as zero for all the operations on the disk. Generally it is used for testing and to fill the gaps in new logical device.

 Error
The error dm target causes any I/O to the mapped disk to fail.  This also is useful for defining gaps in the new logical device.

Such device mapper target can be inserted into the kernel as a module and removed as per wish of the user. Or it can also be inserted into the kernel by creating its patch.Device mapper creates a logical layer of block devices and maps all the I/O requests on this logical layer to underlying existing block devices. For such mapping, device mapper uses a data structure called device mapper table. This table tells us how each and every sector (of size512 bytes) of a logical layer is mapped to a sector on underlying disk. Thus, each target carries out its functionality by doing I/O mapping using its corresponding device mapper table.

So last but not least, Device mapper target represents a type of block device.
Now the " ability to define the type of  block device is provided by device mapper"  so that is why it is generic layer.
So when we want to provide a new type of block device with some advance functionality like snapshots, deduplication. we create new device mapper target, we write logic of the new functionality into that device mapper target.And we can create block devices of that device mapper target. (i.e new type of device ).

Writing Your Own Device Mapper Target

Device Mapper

In the Linux kernel, the device-mapper serves as a generic framework to map a virtual layer of block devices onto existing block devices. The device mapper framework promotes a clean separation of policy and mechanism between user and kernel space respectively. It forms the foundation of LVM2 and EVMS, software RAIDs, dm-crypt disk encryption, and offers additional features such as file-system snapshots. Device-mapper works by processing data passed in from virtual block devices, that it itself provides, and then passing the resultant data on to existing block devices.

"Device mapper can be defined as a generic way to add required functionality in the storage stack by creating virtual layer of block devices and mapping them to existing block devices ". It create virtual layers of block devices that can do different things on top of existing underlying block devices like striping, concatenation, mirroring, snapshot, etc.

The Device Mapper is a modular kernel driver that provides a generic framework for volume management. It has been introduced in the kernel version 2.6. The device-mapper is used by the LVM2 and EVMS 2.x tools.

Device Mapper Target

As stated above, we can create various logical layers through device mapper to carry out the required functionality. Each such layer is created by defining “a device mapper target” for that layer.

There is one to one correspondence between a virtual layer at device mapper layer and the dm target for that layer. The particular dm target contains the code which does the task of implementing functionality which the virtual layer intends to do. For example, a device mapper target can be written to implement mirroring over existing block devices. This dm target shows a virtual layer to upper layers which do the task of mirroring.

Currently seven such features have been added to device mapper through device mapper targets. The device mapper targets are as follows:

• Linear 
• RAID-0 / Striped 
• RAID-1 / Mirrored RAID 
• Snapshot 
• DM-Crypt

Write Our Own Device Mapper Target

Our device mapper target is going to be a kernel module. Lets say we call our dm target as 'basic_target'
and the corresponding file is basic_target.c.

basic_target.c

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <linux/bio.h> #include <linux/device-mapper.h>

/* This is a structure which will store  information about the underlying device 
*  Param:
* dev : underlying device
* start:  Starting sector number of the device
*/

struct my_dm_target {

struct dm_dev *dev;

sector_t start;

};

/* This is map function of basic target. This function gets called whenever you get a new bio
 * request.The working of map function is to map a particular bio request to the underlying device. 
 *The request that we receive is submitted to out device so  bio->bi_bdev points to our device.
 * We should point to the bio-> bi_dev field to bdev of underlying device. Here in this function,
 * we can have other processing like changing sector number of bio request, splitting bio etc. 
 *
 *  Param : 
 *  ti : It is the dm_target structure representing our basic target
 *  bio : The block I/O request from upper layer
 *  map_context : Its mapping context of target.
 *
 *: Return values from target map function:
 *  DM_MAPIO_SUBMITTED :  Your target has submitted the bio request to underlying request
 *  DM_MAPIO_REMAPPED  :  Bio request is remapped, Device mapper should submit bio.  
 *  DM_MAPIO_REQUEUE   :  Some problem has happened with the mapping of bio, So 
 *                                                re queue the bio request. So the bio will be submitted 
 *                                                to the map function  
 */

static int basic_target_map(struct dm_target *ti, struct bio *bio,union map_info *map_context)

{

struct my_dm_target *mdt = (struct my_dm_target *) ti->private;

printk(KERN_CRIT "\n<<in function basic_target_map \n");

bio->bi_bdev = mdt->dev->bdev;

if((bio->bi_rw & WRITE) == WRITE)

printk(KERN_CRIT "\n basic_target_map : bio is a write request.... \n");

else

printk(KERN_CRIT "\n basic_target_map : bio is a read request.... \n");

submit_bio(bio->bi_rw,bio);

printk(KERN_CRIT "\n>>out function basic_target_map \n");

return DM_MAPIO_SUBMITTED;

}

/* This is Constructor Function of basic target * Constructor gets called when we create some device of type 'basic_target'. * So it will get called when we execute command 'dmsetup create' * This function gets called for each device over which you want to create basic * target. Here it is just a basic target so it will take only one device so it * will get called once. */

static int basic_target_ctr(struct dm_target *ti,unsigned int argc,char **argv)

{

struct my_dm_target *mdt;

unsigned long long start;

printk(KERN_CRIT "\n >>in function basic_target_ctr \n");

if (argc != 2) {

printk(KERN_CRIT "\n Invalid no.of arguments.\n");

ti->error = "Invalid argument count";

return -EINVAL;

}

mdt = kmalloc(sizeof(struct my_dm_target), GFP_KERNEL);

if(mdt==NULL)

{

printk(KERN_CRIT "\n Mdt is null\n");

ti->error = "dm-basic_target: Cannot allocate linear context";

return -ENOMEM;

}

if(sscanf(argv[1], "%llu", &start)!=1)

{

ti->error = "dm-basic_target: Invalid device sector";

goto bad;

}

mdt->start=(sector_t)start;

/* dm_get_table_mode * Gives out you the Permissions of device mapper table. * This table is nothing but the table which gets created * when we execute dmsetup create. This is one of the * Data structure used by device mapper for keeping track of its devices. * * dm_get_device * The function sets the mdt->dev field to underlying device dev structure. */

if (dm_get_device(ti, argv[0], dm_table_get_mode(ti->table), &mdt->dev)) {

ti->error = "dm-basic_target: Device lookup failed";

goto bad;

}

ti->private = mdt;

printk(KERN_CRIT "\n>>out function basic_target_ctr \n");

return 0;

bad:

kfree(mdt);

printk(KERN_CRIT "\n>>out function basic_target_ctr with errorrrrrrrrrr \n");

return -EINVAL;

}

/* * This is destruction function * This gets called when we remove a device of type basic target. The function gets * called per device. */

static void basic_target_dtr(struct dm_target *ti)

{

struct my_dm_target *mdt = (struct my_dm_target *) ti->private;

printk(KERN_CRIT "\n<<in function basic_target_dtr \n");

dm_put_device(ti, mdt->dev);

kfree(mdt);

printk(KERN_CRIT "\n>>out function basic_target_dtr \n");

}

/* * This structure is fops for basic target. */

static struct target_type basic_target = {

.name = "basic_target",

.version = {1,0,0},

.module = THIS_MODULE,

.ctr = basic_target_ctr,

.dtr = basic_target_dtr,

.map = basic_target_map,

};

/*---------Module Functions -----------------*/

static int init_basic_target(void)

{

int result;

result = dm_register_target(&basic_target);

if(result < 0)

printk(KERN_CRIT "\n Error in registering target \n");

return 0;

}

static void cleanup_basic_target(void)

{

dm_unregister_target(&basic_target);

}

module_init(init_basic_target);

module_exit(cleanup_basic_target);

MODULE_LICENSE("GPL");

Makefile 

obj-m +=basic_target.o

all:

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean USAGE TBD  http://linuxgazette.net/114/kapil.html Stay tuned to http://techgmm.blogspot.in/

Thanks !!!