========================================
-zram: Compressed RAM based block devices
+zram: Compressed RAM-based block devices
========================================
Introduction
============
-The zram module creates RAM based block devices named /dev/zram<id>
+The zram module creates RAM-based block devices named /dev/zram<id>
(<id> = 0, 1, ...). Pages written to these disks are compressed and stored
in memory itself. These disks allow very fast I/O and compression provides
-good amounts of memory savings. Some of the usecases include /tmp storage,
-use as swap disks, various caches under /var and maybe many more :)
+good amounts of memory savings. Some of the use cases include /tmp storage,
+use as swap disks, various caches under /var and maybe many more. :)
Statistics for individual zram devices are exported through sysfs nodes at
/sys/block/zram<id>/
======== =============================================================
-EBUSY an attempt to modify an attribute that cannot be changed once
- the device has been initialised. Please reset device first;
+ the device has been initialised. Please reset device first.
-ENOMEM zram was not able to allocate enough memory to fulfil your
- needs;
+ needs.
-EINVAL invalid input has been provided.
======== =============================================================
-If you use 'echo', the returned value that is changed by 'echo' utility,
+If you use 'echo', the returned value is set by the 'echo' utility,
and, in general case, something like::
echo 3 > /sys/block/zram0/max_comp_streams
- if [ $? -ne 0 ];
+ if [ $? -ne 0 ]; then
handle_error
fi
::
modprobe zram num_devices=4
- This creates 4 devices: /dev/zram{0,1,2,3}
+
+This creates 4 devices: /dev/zram{0,1,2,3}
num_devices parameter is optional and tells zram how many devices should be
pre-created. Default: 1.
2) Set max number of compression streams
========================================
-Regardless the value passed to this attribute, ZRAM will always
-allocate multiple compression streams - one per online CPUs - thus
+Regardless of the value passed to this attribute, ZRAM will always
+allocate multiple compression streams - one per online CPU - thus
allowing several concurrent compression operations. The number of
allocated compression streams goes down when some of the CPUs
become offline. There is no single-compression-stream mode anymore,
-unless you are running a UP system or has only 1 CPU online.
+unless you are running a UP system or have only 1 CPU online.
To find out how many streams are currently available::
Using comp_algorithm device attribute one can see available and
currently selected (shown in square brackets) compression algorithms,
-change selected compression algorithm (once the device is initialised
+or change the selected compression algorithm (once the device is initialised
there is no way to change compression algorithm).
Examples::
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.
+In order to add a new /dev/zramX device, perform a read operation on the hot_add
+attribute. This will return either the new device's device id (meaning that you
+can use /dev/zram<id>) or an error code.
Example::
Per-device statistics are exported as various nodes under /sys/block/zram<id>/
-A brief description of exported device attributes. For more details please
-read Documentation/ABI/testing/sysfs-block-zram.
+A brief description of exported device attributes follows. For more details
+please read Documentation/ABI/testing/sysfs-block-zram.
====================== ====== ===============================================
Name access description
File /sys/block/zram<id>/mm_stat
-The stat file represents device's mm statistics. It consists of a single
+The mm_stat file represents the device's mm statistics. It consists of a single
line of text and contains the following stats separated by whitespace:
================ =============================================================
Unit: bytes
mem_limit the maximum amount of memory ZRAM can use to store
the compressed data
- mem_used_max the maximum amount of memory zram have consumed to
+ mem_used_max the maximum amount of memory zram has consumed to
store the data
same_pages the number of same element filled pages written to this disk.
No memory is allocated for such pages.
File /sys/block/zram<id>/bd_stat
-The stat file represents device's backing device statistics. It consists of
+The bd_stat file represents a device's backing device statistics. It consists of
a single line of text and contains the following stats separated by whitespace:
============== =============================================================
echo /dev/sda5 > /sys/block/zramX/backing_dev
before disksize setting. It supports only partition at this moment.
-If admin want to use incompressible page writeback, they could do via::
+If admin wants to use incompressible page writeback, they could do via::
- echo huge > /sys/block/zramX/write
+ echo huge > /sys/block/zramX/writeback
To use idle page writeback, first, user need to declare zram pages
as idle::
echo all > /sys/block/zramX/idle
From now on, any pages on zram are idle pages. The idle mark
-will be removed until someone request access of the block.
+will be removed until someone requests access of the block.
IOW, unless there is access request, those pages are still idle pages.
Admin can request writeback of those idle pages at right timing via::
To overcome the concern, zram supports "writeback_limit" feature.
The "writeback_limit_enable"'s default value is 0 so that it doesn't limit
-any writeback. IOW, if admin want to apply writeback budget, he should
+any writeback. IOW, if admin wants to apply writeback budget, he should
enable writeback_limit_enable via::
$ echo 1 > /sys/block/zramX/writeback_limit_enable
Once writeback_limit_enable is set, zram doesn't allow any writeback
-until admin set the budget via /sys/block/zramX/writeback_limit.
+until admin sets the budget via /sys/block/zramX/writeback_limit.
(If admin doesn't enable writeback_limit_enable, writeback_limit's value
-assigned via /sys/block/zramX/writeback_limit is meaninless.)
+assigned via /sys/block/zramX/writeback_limit is meaningless.)
If admin want to limit writeback as per-day 400M, he could do it
like below::
/sys/block/zram0/writeback_limit.
$ echo 1 > /sys/block/zram0/writeback_limit_enable
-If admin want to allow further write again once the bugdet is exausted,
+If admins want to allow further write again once the bugdet is exhausted,
he could do it like below::
$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
/sys/block/zram0/writeback_limit
-If admin want to see remaining writeback budget since he set::
+If admin wants to see remaining writeback budget since last set::
$ cat /sys/block/zramX/writeback_limit
$ echo 0 > /sys/block/zramX/writeback_limit_enable
-The writeback_limit count will reset whenever you reset zram(e.g.,
+The writeback_limit count will reset whenever you reset zram (e.g.,
system reboot, echo 1 > /sys/block/zramX/reset) so keeping how many of
writeback happened until you reset the zram to allocate extra writeback
budget in next setting is user's job.
-If admin want to measure writeback count in a certain period, he could
+If admin wants to measure writeback count in a certain period, he could
know it via /sys/block/zram0/bd_stat's 3rd column.
memory tracking
git.samba.org / sfrench / cifs-2.6.git / blobdiff