zram-config

Overview

This is a complete zram-config utility for swap, directories, and logs to reduce SD, NAND and eMMC block wear. Furthermore zram allows near RAM speed access to working directories, and prevents frequent writing to persistent storage. Even more importantly, data stored in zram can be compressed to conserve memory.

A table located at /etc/ztab is used to configure any number and type of zram devices. Using the table an OverlayFS mount is used to mount the newly created zram device as the upper filesystem of the OverlayFS. OverlayFS is used so that files do not need to be copied from persistent storage to RAM on startup. In theory this should allow for faster boots and larger directories as no complete directory copy is needed. A modified version of kmxz/overlayfs-tools is used to implement the OverlayFS sync logic.

This tool is primarily developed and tested against Raspberry Pi OS. Any Debian derivative should also work out of the box, however there is no guarantee. Experimental Alpine support has also been added, other distributions may work but once again, there is no guarantee.

A Brief Usage Guide

Table of Contents

1. Install
   • Manually start or stop
   • Sync files to disk
2. Update
3. Uninstall
4. Configure
   • Example configuration
5. Is it working?
6. Known issues
   • Conflicts with services
   • Swapiness on older Linux kernel versions
   • Raspberry Pi 4 8GB compatibility
7. Performance
8. Reference

Install

sudo apt-get install git
git clone https://github.com/ecdye/zram-config
sudo ./zram-config/install.bash

Manually start or stop

On Debian, use sudo systemctl {start|stop} zram-config.service to start or stop zram-config. On Alpine, use sudo rc-service zram-config {start|stop}. This will ensure that any changes are properly synced to the persistent storage before system poweroff.

Sync files to disk

Run sudo zram-config sync to sync any changes in the zram filesystems managed by zram-config to persistent storage. If you have concerns about losing data due to sudden power loss you could use this to ensure that changes are synced to disk periodically.

A default sync service that will sync files to disk every night can be installed by running the following.

sudo /path/to/zram-config/install.bash sync

Note that this sync service is not installed by default, you must install it separately.

Update

sudo /path/to/zram-config/update.bash

Uninstall

sudo /usr/local/share/zram-config/uninstall.bash

Configure

All configuration is done in the /etc/ztab file.

Use # to comment out any line, add new drives with the first column providing the drive type and then drive details separated by tab characters.

All algorithms in /proc/crypto are supported but only lzo-rle, lzo, lz4, and zstd have zramctl text strings; lzo-rle is the fastest with zstd having much better text compression.

mem_limit is the compressed memory limit and will set a hard memory limit for the system admin. Set to 0 to disable the mem_limit.

disk_size is the maximum size of the uncompressed memory. It should be set to roughly 150% of mem_limit depending on the algorithm and how compressible the input files are. Don't make it much higher than the compression algorithm (and the additional zram overhead) is capable of because there is a ~0.1% memory overhead when empty.

swap_priority will set zram over alternative swap devices.

page-cluster 0 means tuning to singular pages rather than the default 3 which caches 8 for HDD tuning, which can lower latency.

swappiness 150 because the improved performance of zram allows more usage without any adverse affects from the default of 60. It can be raised up to 200 which will improve performance in high memory pressure situations.

target_dir is the directory you wish to hold in zram, and the original will be moved to a bind mount bind_dir and is synchronized on start, stop, and write commands.

bind_dir is the directory where the original directory will be mounted for sync purposes. Usually in /opt or /var, name optional.

oldlog_dir will enable log-rotation to an off device directory while retaining only live logs in zram. Usually in /opt or /var, name optional.

If you need multiple zram swaps or zram directories, just create another entry in /etc/ztab. To do this simply add the new entries to the /etc/ztab, if you need to edit an active zram device you must stop zram with sudo systemctl stop zram-config.service on Debian or sudo rc-service zram-config stop on Alpine and then edit any entries you need to. Once finished, start zram using sudo systemctl start zram-config.service or sudo rc-service zram-config start which will only add the new entries if zram is already running.

Example configuration

# swap	alg		mem_limit	disk_size	swap_priority	page-cluster	swappiness
swap	lzo-rle		250M		750M		75		0		150

# dir	alg		mem_limit	disk_size	target_dir		bind_dir
#dir	lzo-rle		50M		150M		/home/pi		/opt/zram/pi.bind

# log	alg		mem_limit	disk_size	target_dir		bind_dir		oldlog_dir
log	lzo-rle		50M		150M		/var/log		/opt/zram/log.bind	/opt/zram/oldlog

Is it working?

Run zramctl in your preferred shell and if you see and output similar to below, yes it is working. Please note that if the zramctl command is missing, you will need to install the util-linux package to have a convenient way to view the zram status.

pi@raspberrypi:~$ zramctl
NAME       ALGORITHM DISKSIZE  DATA  COMPR TOTAL STREAMS MOUNTPOINT
/dev/zram1 lzo-rle       150M 16.9M 373.2K  692K       4 /opt/zram/zram1
/dev/zram0 lzo-rle       750M    4K    87B   12K       4 [SWAP]

To view more information on zram usage take a look at the following commands and their corresponding output.

pi@raspberrypi:~$ df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/root        3833792 1368488   2275172  38% /
devtmpfs          437356       0    437356   0% /dev
tmpfs             471980       0    471980   0% /dev/shm
tmpfs             188792     440    188352   1% /run
tmpfs               5120       0      5120   0% /run/lock
/dev/mmcblk0p1    258095   49436    208660  20% /boot
/dev/zram1        132240   18440    103048  16% /opt/zram/zram1
overlay1          132240   18440    103048  16% /var/log
tmpfs              94396       0     94396   0% /run/user/1000

pi@raspberrypi:~$ free -h
               total        used        free      shared  buff/cache   available
Mem:           921Mi        46Mi       750Mi       0.0Ki       124Mi       819Mi
Swap:          849Mi          0B       849Mi

pi@raspberrypi:~$ swapon
NAME       TYPE      SIZE USED PRIO
/var/swap  file      100M   0B   -2
/dev/zram0 partition 750M   0B   75

Known issues

Conflicts with services

When running zram on a directory that has services accessing it, they will need to be stopped before starting or stopping zram. For example, in the log zram device zram-config stops the services that run by default in the /var/log directory before starting or stopping. If your system has other services that write to /var/log that are not stopped zram may fail to properly sync files and remove the zram device when stopping, and will probably outright fail to start when initializing a zram device. This issue is not limited to logs, if you are running zram on another directoy that is written to by a service you will run into the same issue.

For an example on how this project internally takes care of this issue see the serviceConfiguration function in zram-config. A more in depth version of this function is used in the openHAB branch that can be referenced as well.

Swapiness on older Linux kernel versions

When running zram swap on Linux kernel versions older than 5.8 swappiness has a maximum value of 100. If you observe issues runnning on older kernel versions try setting the default value of 150 back to 100.

Raspberry Pi 4 8GB compatibility

The Raspberry Pi 4 8GB model can exhibit issues with zram due to a Linux kernel bug. This bug has been fixed as of Raspberry Pi Kernel version 1.20210527. See raspberrypi/linux@cef3970381 for more details about the issue.

Performance

LZO-RLE offers the best performance and is probably the best choice, and from kernel 5.1 and onward it is the default. If you are not running at least kernel 5.1 then LZO-RLE may not be supported by your system and you may need to change /etc/ztab accordingly. You might have text based low impact directories such as /var/log or /var/cache where a highly effective text compressor such as zstd is optimal, with effective compression that can be up to 200% of what LZO may achieve especially with text. With /tmp and /run, zram is unnecessary because they are RAM mounted as tmpfs and, if memory gets short, then the zram swap will provide extra. It is only under intense loads that the slight overhead of zram compression becomes noticeable.

This chart from facebook/zstd provides a good benchmark for the performance of the different compressors.

Reference

https://www.kernel.org/doc/Documentation/blockdev/zram.txt

https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt