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diff --git a/Documentation/blockdev/loop.rst b/Documentation/blockdev/loop.rst new file mode 100644 index 000000000000..a11457479cb4 --- /dev/null +++ b/Documentation/blockdev/loop.rst @@ -0,0 +1,83 @@ +Loopback Block Device +===================== + +Overview +-------- + +The loopback device driver allows you to use a regular file as a block device. +You can then create a file system on that block device and mount it just as you +would mount other block devices such as hard drive partitions, CD-ROM drives or +floppy drives. The loop devices are block special device files with major +number 7 and typically called /dev/loop0, /dev/loop1 etc. + +To use the loop device, you need the losetup utility, found in the `util-linux +package <https://www.kernel.org/pub/linux/utils/util-linux/>`_. + +.. note:: + Note that this loop device has nothing to do with the loopback device \ + used for network connections from the machine to itself. + + +Parameters +---------- + +Kernel Command Line Parameters +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + + max_loop + The number of loop block devices that get unconditionally + pre-created at init time. The default number is configured by + BLK_DEV_LOOP_MIN_COUNT. Instead of statically allocating a + predefined number, loop devices can be requested on-demand + with the /dev/loop-control interface. + + +Module parameters +~~~~~~~~~~~~~~~~~ + + max_part + Maximum number of partitions per loop device (default: 0). + + If max_part is given, partition scanning is globally enabled + for all loop devices. + + max_loop + Maximum number of loop devices that should be initialized + (default: 8). The default number is configured by + BLK_DEV_LOOP_MIN_COUNT. + + +File format drivers +------------------- + +The loopback device driver provides an interface for kernel modules to +implement custom file formats. By default, an initialized loop device uses the +**RAW** file format driver. + +.. note:: + If you want to create and set up a new loop device with the losetup \ + utility make sure that the suitable file format driver is loaded \ + before. + +The following file format drivers are available. + + +RAW +~~~ + +The RAW file format driver implements the binary reading and writing of a disk +image file. It supports discarding, asynchrounous IO, flushing and cryptoloop +support. + +The driver's kernel module is named *loop_file_fmt_raw*. + + +QCOW +~~~~ + +The QCOW file format driver implements QEMU's copy on write file format in +version 2. At the moment, the file format driver only supports the reading +of QCOW2 disk image files. It does not support writing to QCOW2 images, the +recovery of broken QCOW images, snapshots and reference counts. + +The driver's kernel module is named *loop_file_fmt_qcow*. |