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Bản gốc:: zram: Compressed RAM-based block devices
Người dịch:: Google Translate (machine translation)
Phiên bản gốc:: 8541d8f725c6

Cảnh báo

Tài liệu này được dịch tự động bằng máy và chưa được review bởi người dịch. Nội dung có thể không chính xác hoặc khó hiểu ở một số chỗ. Khi có sự khác biệt với bản gốc, bản gốc luôn là chuẩn. Bản dịch chất lượng cao (được review) được đặt trong thư mục vi_VN/.

zram: Thiết bị khối dựa trên RAM được nén¶

Giới thiệu¶

Mô-đun zram tạo các thiết bị khối dựa trên RAM có tên /dev/zram<id> (<id> = 0, 1, ...). Các trang được ghi vào các đĩa này được nén và lưu trữ trong chính bộ nhớ của mình. Những đĩa này cho phép I/O rất nhanh và cung cấp khả năng nén lượng tiết kiệm bộ nhớ tốt. Một số trường hợp sử dụng bao gồm lưu trữ /tmp, sử dụng làm đĩa trao đổi, các bộ đệm khác nhau trong /var và có thể nhiều hơn nữa. :)

Thống kê cho các thiết bị zram riêng lẻ được xuất qua các nút sysfs tại /sys/block/zram<id>/

Cách sử dụng¶

Có một số cách để định cấu hình và quản lý (-s) thiết bị zram:

sử dụng thuộc tính sysfs zram và zram_control
sử dụng tiện ích zramctl, được cung cấp bởi util-linux (util-linux@vger.kernel.org).

Trong tài liệu này, chúng tôi sẽ chỉ mô tả các bước cấu hình zram ‘thủ công’, Thuộc tính sysfs IOW, zram và zram_control.

Để hiểu rõ hơn về zramctl, vui lòng tham khảo util-linux tài liệu, trang man zramctl hoặc ZZ0000ZZ. Xin được thông báo rằng những người bảo trì zram không phát triển/duy trì util-linux hoặc zramctl, nên bạn có thắc mắc vui lòng liên hệ util-linux@vger.kernel.org

Sau đây trình bày trình tự các bước điển hình để sử dụng zram.

WARNING¶

Để đơn giản, chúng tôi bỏ qua phần kiểm tra lỗi trong hầu hết các ví dụ dưới đây. Tuy nhiên, trách nhiệm duy nhất của bạn là xử lý lỗi.

Thuộc tính zram sysfs luôn trả về giá trị âm trong trường hợp có lỗi. Danh sách các mã trả lại có thể:

-ENOMEM zram không thể phân bổ đủ bộ nhớ để đáp ứng nhu cầu của bạn: nhu cầu.

-EINVAL đầu vào không hợp lệ đã được cung cấp. -EAGAIN thử lại thao tác sau (ví dụ: khi cố chạy giải nén

và viết lại đồng thời).

Nếu bạn sử dụng ‘echo’, giá trị trả về được thiết lập bởi tiện ích ‘echo’, và, trong trường hợp chung, đại loại như:

echo foo > /sys/block/zram0/comp_algorithm

nếu [ $? -ne 0 ]; sau đó: xử lý_lỗi

fi

nên đủ.

1) Mô-đun tải¶

modprobe zram num_devices=4

Điều này tạo ra 4 thiết bị: /dev/zram{0,1,2,3}

Tham số num_devices là tùy chọn và cho zram biết cần có bao nhiêu thiết bị được tạo trước. Mặc định: 1.

2) Chọn thuật toán nén¶

Using comp_algorithm device attribute one can see available and currently selected (shown in square brackets) compression algorithms, or change the selected compression algorithm (once the device is initialised there is no way to change compression algorithm).

Examples:

#show supported compression algorithms
cat /sys/block/zram0/comp_algorithm
lzo [lz4]

#select lzo compression algorithm
echo lzo > /sys/block/zram0/comp_algorithm

For the time being, the comp_algorithm content shows only compression algorithms that are supported by zram.

3) Set compression algorithm parameters: Optional¶

Compression algorithms may support specific parameters which can be tweaked for particular dataset. ZRAM has an algorithm_params device attribute which provides a per-algorithm params configuration.

For example, several compression algorithms support level parameter. In addition, certain compression algorithms support pre-trained dictionaries, which significantly change algorithms’ characteristics. In order to configure compression algorithm to use external pre-trained dictionary, pass full path to the dict along with other parameters:

#pass path to pre-trained zstd dictionary
echo "algo=zstd dict=/etc/dictionary" > /sys/block/zram0/algorithm_params

#same, but using algorithm priority
echo "priority=1 dict=/etc/dictionary" > \
        /sys/block/zram0/algorithm_params

#pass path to pre-trained zstd dictionary and compression level
echo "algo=zstd level=8 dict=/etc/dictionary" > \
        /sys/block/zram0/algorithm_params

Parameters are algorithm specific: not all algorithms support pre-trained dictionaries, not all algorithms support level. Furthermore, for certain algorithms level controls the compression level (the higher the value the better the compression ratio, it even can take negatives values for some algorithms), for other algorithms level is acceleration level (the higher the value the lower the compression ratio).

4) Set Disksize¶

Set disk size by writing the value to sysfs node ‘disksize’. The value can be either in bytes or you can use mem suffixes. Examples:

# Initialize /dev/zram0 with 50MB disksize
echo $((50*1024*1024)) > /sys/block/zram0/disksize

# Using mem suffixes
echo 256K > /sys/block/zram0/disksize
echo 512M > /sys/block/zram0/disksize
echo 1G > /sys/block/zram0/disksize

Note: There is little point creating a zram of greater than twice the size of memory since we expect a 2:1 compression ratio. Note that zram uses about 0.1% of the size of the disk when not in use so a huge zram is wasteful.

5) Set memory limit: Optional¶

Set memory limit by writing the value to sysfs node ‘mem_limit’. The value can be either in bytes or you can use mem suffixes. In addition, you could change the value in runtime. Examples:

# limit /dev/zram0 with 50MB memory
echo $((50*1024*1024)) > /sys/block/zram0/mem_limit

# Using mem suffixes
echo 256K > /sys/block/zram0/mem_limit
echo 512M > /sys/block/zram0/mem_limit
echo 1G > /sys/block/zram0/mem_limit

# To disable memory limit
echo 0 > /sys/block/zram0/mem_limit

6) Activate¶

mkswap /dev/zram0
swapon /dev/zram0

mkfs.ext4 /dev/zram1
mount /dev/zram1 /tmp

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 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:

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 attributes follows. For more details please read ABI file testing/sysfs-block-zram.

Name	access	description
disksize	RW	show and set the device’s disk size
initstate	RO	shows the initialization state of the device
reset	WO	trigger device reset
mem_used_max	WO	reset the mem_used_max counter (see later)
mem_limit	WO	specifies the maximum amount of memory ZRAM can use to store the compressed data
writeback_limit	WO	specifies the maximum amount of write IO zram can write out to backing device as 4KB unit
writeback_limit_enable	RW	show and set writeback_limit feature
writeback_batch_size	RW	show and set maximum number of in-flight writeback operations
compressed_writeback	RW	show and set compressed writeback feature
comp_algorithm	RW	show and change the compression algorithm
algorithm_params	WO	setup compression algorithm parameters
compact	WO	trigger memory compaction
debug_stat	RO	this file is used for zram debugging purposes
backing_dev	RW	set up backend storage for zram to write out
idle	WO	mark allocated slot as idle

User space is advised to use the following files to read the device statistics.

File /sys/block/zram<id>/stat

Represents block layer statistics. Read Block layer statistics in /sys/block/<dev>/stat for details.

File /sys/block/zram<id>/io_stat

The stat file represents device’s I/O statistics not accounted by block layer and, thus, not available in zram<id>/stat file. It consists of a single line of text and contains the following stats separated by whitespace:

failed_reads

The number of failed reads

failed_writes

The number of failed writes

invalid_io

The number of non-page-size-aligned I/O requests

notify_free

Depending on device usage scenario it may account

the number of pages freed because of swap slot free notifications

the number of pages freed because of REQ_OP_DISCARD requests sent by bio. The former ones are sent to a swap block device when a swap slot is freed, which implies that this disk is being used as a swap disk.

The latter ones are sent by filesystem mounted with discard option, whenever some data blocks are getting discarded.

File /sys/block/zram<id>/mm_stat

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:

orig_data_size

uncompressed size of data stored in this disk. Unit: bytes

compr_data_size

compressed size of data stored in this disk

mem_used_total

the amount of memory allocated for this disk. This includes allocator fragmentation and metadata overhead, allocated for this disk. So, allocator space efficiency can be calculated using compr_data_size and this statistic. 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 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.

pages_compacted

the number of pages freed during compaction

huge_pages

the number of incompressible pages

huge_pages_since

the number of incompressible pages since zram set up

File /sys/block/zram<id>/bd_stat

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:

bd_count

size of data written in backing device. Unit: 4K bytes

bd_reads

the number of reads from backing device Unit: 4K bytes

bd_writes

the number of writes to backing device Unit: 4K bytes

9) Deactivate¶

swapoff /dev/zram0
umount /dev/zram1

10) Reset¶

Write any positive value to ‘reset’ sysfs node:
echo 1 > /sys/block/zram0/reset
echo 1 > /sys/block/zram1/reset
This frees all the memory allocated for the given device and resets the disksize to zero. You must set the disksize again before reusing the device.

Optional Feature¶

IDLE pages tracking¶

zram has built-in support for idle pages tracking (that is, allocated but not used pages). This feature is useful for e.g. zram writeback and recompression. In order to mark pages as idle, execute the following command:

echo all > /sys/block/zramX/idle

This will mark all allocated zram pages as idle. The idle mark will be removed only when the page (block) is accessed (e.g. overwritten or freed). Additionally, when CONFIG_ZRAM_TRACK_ENTRY_ACTIME is enabled, pages can be marked as idle based on how many seconds have passed since the last access to a particular zram page:

echo 86400 > /sys/block/zramX/idle

In this example, all pages which haven’t been accessed in more than 86400 seconds (one day) will be marked idle.

writeback¶

With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page to backing storage rather than keeping it in memory. To use the feature, admin should set up backing device via:

echo /dev/sda5 > /sys/block/zramX/backing_dev

before disksize setting. It supports only partitions at this moment. If admin wants to use incompressible page writeback, they could do it via:

echo huge > /sys/block/zramX/writeback

Admin can request writeback of idle pages at right timing via:

echo idle > /sys/block/zramX/writeback

With the command, zram will writeback idle pages from memory to the storage.

Additionally, if a user choose to writeback only huge and idle pages this can be accomplished with:

echo huge_idle > /sys/block/zramX/writeback

If a user chooses to writeback only incompressible pages (pages that none of algorithms can compress) this can be accomplished with:

echo incompressible > /sys/block/zramX/writeback

If an admin wants to write a specific page in zram device to the backing device, they could write a page index into the interface:

echo "page_index=1251" > /sys/block/zramX/writeback

In Linux 6.16 this interface underwent some rework. First, the interface now supports key=value format for all of its parameters (type=huge_idle, etc.) Second, the support for page_indexes was introduced, which specify LOW-HIGH range (or ranges) of pages to be written-back. This reduces the number of syscalls, but more importantly this enables optimal post-processing target selection strategy. Usage example:

echo "type=idle" > /sys/block/zramX/writeback
echo "page_indexes=1-100 page_indexes=200-300" > \
        /sys/block/zramX/writeback

We also now permit multiple page_index params per call and a mix of single pages and page ranges:

echo page_index=42 page_index=99 page_indexes=100-200 \
        page_indexes=500-700 > /sys/block/zramX/writeback

If there are lots of write IO with flash device, potentially, it has flash wearout problem so that admin needs to design write limitation to guarantee storage health for entire product life.

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 wants to apply writeback budget, they 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 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 meaningless.)

If admin wants to limit writeback as per-day 400M, they could do it like below:

$ MB_SHIFT=20
$ 4K_SHIFT=12
$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
        /sys/block/zram0/writeback_limit.
$ echo 1 > /sys/block/zram0/writeback_limit_enable

If admins want to allow further write again once the budget is exhausted, they could do it like below:

$ echo $((400<<MB_SHIFT>>4K_SHIFT)) > \
        /sys/block/zram0/writeback_limit

If an admin wants to see the remaining writeback budget since last set:

$ cat /sys/block/zramX/writeback_limit

If an admin wants to disable writeback limit, they could do:

$ echo 0 > /sys/block/zramX/writeback_limit_enable

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.

By default zram stores written back pages in decompressed (raw) form, which means that writeback operation involves decompression of the page before writing it to the backing device. This behavior can be changed by enabling compressed_writeback feature, which causes zram to write compressed pages to the backing device, thus avoiding decompression overhead. To enable this feature, execute:

$ echo yes > /sys/block/zramX/compressed_writeback

Note that this feature should be configured before the zramX device is initialized.

Depending on backing device storage type, writeback operation may benefit from a higher number of in-flight write requests (batched writes). The number of maximum in-flight writeback operations can be configured via writeback_batch_size attribute. To change the default value (which is 32), execute:

$ echo 64 > /sys/block/zramX/writeback_batch_size

If admin wants to measure writeback count in a certain period, they could know it via /sys/block/zram0/bd_stat’s 3rd column.

recompression¶

With CONFIG_ZRAM_MULTI_COMP, zram can recompress pages using alternative (secondary) compression algorithms. The basic idea is that alternative compression algorithm can provide better compression ratio at a price of (potentially) slower compression/decompression speeds. Alternative compression algorithm can, for example, be more successful compressing huge pages (those that default algorithm failed to compress). Another application is idle pages recompression - pages that are cold and sit in the memory can be recompressed using more effective algorithm and, hence, reduce zsmalloc memory usage.

With CONFIG_ZRAM_MULTI_COMP, zram supports up to 4 compression algorithms: one primary and up to 3 secondary ones. Primary zram compressor is explained in “3) Select compression algorithm”, secondary algorithms are configured using recomp_algorithm device attribute.

Example::

#show supported recompression algorithms
cat /sys/block/zramX/recomp_algorithm
#1: lzo lzo-rle lz4 lz4hc [zstd]
#2: lzo lzo-rle lz4 [lz4hc] zstd

Alternative compression algorithms are sorted by priority. In the example above, zstd is used as the first alternative algorithm, which has priority of 1, while lz4hc is configured as a compression algorithm with priority 2. Alternative compression algorithm’s priority is provided during algorithms configuration::

#select zstd recompression algorithm, priority 1
echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm

#select deflate recompression algorithm, priority 2
echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm

Another device attribute that CONFIG_ZRAM_MULTI_COMP enables is recompress, which controls recompression.

Examples::

#IDLE pages recompression is activated by `idle` mode
echo "type=idle priority=1" > /sys/block/zramX/recompress

#HUGE pages recompression is activated by `huge` mode
echo "type=huge priority=2" > /sys/block/zram0/recompress

#HUGE_IDLE pages recompression is activated by `huge_idle` mode
echo "type=huge_idle priority=1" > /sys/block/zramX/recompress

The number of idle pages can be significant, so user-space can pass a size threshold (in bytes) to the recompress knob: zram will recompress only pages of equal or greater size::

#recompress all pages larger than 3000 bytes
echo "threshold=3000 priority=1" > /sys/block/zramX/recompress

#recompress idle pages larger than 2000 bytes
echo "type=idle threshold=2000 priority=1" > \
        /sys/block/zramX/recompress

It is also possible to limit the number of pages zram re-compression will attempt to recompress::

echo "type=huge_idle priority=1 max_pages=42" > \
        /sys/block/zramX/recompress

It is advised to always specify priority parameter. While it is also possible to specify algo parameter, so that zram will use algorithm’s name to determine the priority, it is not recommended, since it can lead to unexpected results when the same algorithm is configured with different priorities (e.g. different parameters). priority is the only way to guarantee that the expected algorithm will be used.

memory tracking¶

With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the zram block. It could be useful to catch cold or incompressible pages of the process with*pagemap.

If you enable the feature, you could see block state via /sys/kernel/debug/zram/zram0/block_state”. The output is as follows:

  75.033841 .wh...
  63.806904 s.....
  63.806919 ..hi..
  62.801919 ....r.
 146.781902 ..hi.n

First column

zram’s block index.

Second column

access time since the system was booted

Third column

state of the block:

s:: same page
w:: written page to backing store
h:: huge page
i:: idle page
r:: recompressed page (secondary compression algorithm)
n:: none (including secondary) of algorithms could compress it

First line of above example says 300th block is accessed at 75.033841sec and the block’s state is huge so it is written back to the backing storage. It’s a debugging feature so anyone shouldn’t rely on it to work properly.

Nitin Gupta ngupta@vflare.org

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