zram: update documentation

* Usage

There are several ways to configure and manage zram device(-s):

a) using zram and zram_control sysfs attributes

b) using zramctl utility, provided by util-linux (util-linux@vger.kernel.org).

In this document we will describe only 'manual' zram configuration steps,

IOW, zram and zram_control sysfs attributes.

In order to get a better idea about zramctl please consult util-linux

documentation, zramctl man-page or `zramctl --help'. Please be informed

that zram maintainers do not develop/maintain util-linux or zramctl, should

you have any questions please contact util-linux@vger.kernel.org

Following shows a typical sequence of steps for using zram.

WARNING

=======

For the sake of simplicity we skip error checking parts in most of the

examples below. However, it is your sole responsibility to handle errors.

zram sysfs attributes always return negative values in case of errors.

The list of possible return codes:

-EBUSY	-- an attempt to modify an attribute that cannot be changed once

the device has been initialised. Please reset device first;

-ENOMEM	-- zram was not able to allocate enough memory to fulfil your

needs;

-EINVAL	-- invalid input has been provided.

If you use 'echo', the returned value that is changed by 'echo' utility,

and, in general case, something like:

	echo 3 > /sys/block/zram0/max_comp_streams

	if [ $? -ne 0 ];

		handle_error

	fi

should suffice.

1) Load Module:

	modprobe zram num_devices=4

	This creates 4 devices: /dev/zram{0,1,2,3}

3) Select compression algorithm

	Using comp_algorithm device attribute one can see available and

	currently selected (shown in square brackets) compression algortithms,

	currently selected (shown in square brackets) compression algorithms,

	change selected compression algorithm (once the device is initialised

	there is no way to change compression algorithm).

8) Stats:

Per-device statistics are exported as various nodes under /sys/block/zram<id>/

A brief description of exported device attritbutes. For more details please

A brief description of exported device attributes. For more details please

read Documentation/ABI/testing/sysfs-block-zram.

Name            access            description

orig_data_size    RO    uncompressed size of data stored in this disk

compr_data_size   RO    compressed size of data stored in this disk

mem_used_total    RO    the amount of memory allocated for this disk

mem_used_max      RW    the maximum amount memory zram have consumed to

                        store compressed data

mem_used_max      RW    the maximum amount of memory zram have consumed to

                        store the data (to reset this counter to the actual

                        current value, write 1 to this attribute)

mem_limit         RW    the maximum amount of memory ZRAM can use to store

                        the compressed data

pages_compacted   RO    the number of pages freed during compaction