* 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}
- (num_devices parameter is optional. Default: 1)
+
+num_devices parameter is optional and tells zram how many devices should be
+pre-created. Default: 1.
2) Set max number of compression streams
Compression backend may use up to max_comp_streams compression streams,
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).
mkfs.ext4 /dev/zram1
mount /dev/zram1 /tmp
-7) Stats:
+7) Add/remove zram devices
+
+zram provides a control interface, which enables dynamic (on-demand) device
+addition and removal.
+
+In order to add a new /dev/zramX device, perform read operation on hot_add
+attribute. This will return either new device's device id (meaning that you
+can use /dev/zram<id>) or error code.
+
+Example:
+ cat /sys/class/zram-control/hot_add
+ 1
+
+To remove the existing /dev/zramX device (where X is a device id)
+execute
+ echo X > /sys/class/zram-control/hot_remove
+
+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
-num_migrated RO the number of objects migrated migrated by compaction
-
+pages_compacted RO the number of pages freed during compaction
+ (available only via zram<id>/mm_stat node)
+compact WO trigger memory compaction
WARNING
=======
zero_pages
num_migrated
-8) Deactivate:
+9) Deactivate:
swapoff /dev/zram0
umount /dev/zram1
-9) Reset:
+10) Reset:
Write any positive value to 'reset' sysfs node
echo 1 > /sys/block/zram0/reset
echo 1 > /sys/block/zram1/reset
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