zramSwap: allow configure compression algorithm + cleanups #52991

danbst · 2018-12-27T22:22:56Z

Motivation for this change

As I mentioned in https://discourse.nixos.org/t/install-nixos-on-512-mb-ram/1713, I'm using zstd compression, but current module doesn't allow to change compressor declaratively. Here we go, and fix few problems with this module.

Things done

add zramSwap.algorithm option, which allows to change compressor
declaratively
some documentation changes
replaced /sys/block/zram* handling with zramctl, because I had occasional
"Device is busy" error (looks like zram has to be configured in predefined order)
added memoryPercent and algorithm as restart triggers. I think, it was
a bug that changing memoryPercent in configuration wasn't applied immediately.
removed a bind to .swap device. While it looks natural (when swap device goes
off, so should zram device), it wasn't implemented properly. This caused problems
with swapon/swapoff:

$ cat /proc/swaps
Filename                                Type            Size    Used    Priority
/dev/zram0                              partition       8166024 0       -2
/var/swapfile                           file            5119996 5120    1

$ sudo swapoff -a

$ sudo swapon -a
swapon: /dev/zram0: read swap header failed

$ cat /proc/swaps
Filename                                Type            Size    Used    Priority
/var/swapfile                           file            5119996 0       1

cc @Mic92

worldofpeace · 2018-12-27T23:24:14Z

I'm using zstd compression, but current module doesn't allow to change compressor declaratively.

I came across this also, and what I was doing for a while was that I added an algorithm option but used an additional udev rule to implement it

services.udev.extraRules = ''
  KERNEL=="zram[0-9]*", ENV{SYSTEMD_WANTS}="zram-init-%k.service", TAG+="systemd", ATTR{comp_algorithm}="${cfg.algorithm}"
'';

I also think there's a disksize attribute as well.

danbst · 2018-12-28T00:51:49Z

@worldofpeace yeah, according to gist https://gist.github.com/asssaf/f9bef956a755e67ed742e3265765e48d it is possible to do only with udev. However I'm not sure udev solution will apply required changes when options are changed. For example, when you change compressor and rebuild system, does it rebuild zram devices with new algo?

worldofpeace · 2018-12-28T01:28:21Z

@worldofpeace yeah, according to gist https://gist.github.com/asssaf/f9bef956a755e67ed742e3265765e48d it is possible to do only with udev. However I'm not sure udev solution will apply required changes when options are changed. For example, when you change compressor and rebuild system, does it rebuild zram devices with new algo?

It would rebuild the udev rules and I'm pretty sure those are monitored for changes so it would just reapply them once changed.

danbst · 2018-12-28T02:23:28Z

Also, just learned that numDevices description (which I've added) is not actual since linux 3.16 (sick). So maybe I should go and deprecate numDevices altogether (we don't do per-device configuration).

danbst · 2019-01-02T00:36:26Z

Alright, I've fixed things up.

one more old race condition hunted!
replaced integer math with floating math!
introduced swapDevices. In general, zram and swap are not same, you can allocate zram device and use it as regular block device, including compressed tmpfs usage. So, one can set 1 device for swap, and 1 for other things. Previously it required lots of hacking.
for those who use several zram devices added a warning that this may be misusage. The warning can be disabled by explicitly setting zramSwap.swapDevices.
documentation, documentation
and finally, zstd is default now!

cc for review @worldofpeace @infinisil @bjornfor @wmertens @wizeman

nixos-discourse · 2019-01-02T00:47:26Z

This pull request has been mentioned on Nix community. There might be relevant details there:

https://discourse.nixos.org/t/install-nixos-on-512-mb-ram/1713/13

Mic92 · 2019-01-14T17:19:30Z

nixos/modules/config/zram.nix

+        description = ''
+          Compression algorithm. <literal>lzo</literal> has good compression,
+          but is slow. <literal>lz4</literal> has bad compression, but is fast.
+          <literal>zstd</literal> is both good compression and fast (kernel 4.18+).


There could be an assertion generated if the kernel is too old (config.boot.kernelPackages.kernel.version)

I'd better remove this section. Current kernel is already capable of zstd zram.

Mic92 · 2019-01-14T17:21:16Z

nixos/modules/config/zram.nix

-              totalmem=$(${pkgs.gnugrep}/bin/grep 'MemTotal: ' /proc/meminfo | ${pkgs.gawk}/bin/awk '{print $2}')
-              mem=$(((totalmem * ${toString cfg.memoryPercent} / 100 / ${toString cfg.numDevices}) * 1024))
+              totalmem=$(${pkgs.gawk}/bin/awk '/MemTotal: /{print $2}' /proc/meminfo)
+              mem=$(echo "$totalmem*${toString cfg.memoryPercent}/100.0/${toString devicesCount}*1024" | ${bc})


Do we depend on bc in the minimal closure?

Otherwise gawk could also perform floating point calculation I think.

- add `zramSwap.algorithm` option, which allows to change compressor declaratively. zstd as default - add `zramSwap.swapDevices` option, which allows to define how many zram devices will be used as swap. Rest devices can be managed freely - simpler floating calculations - fix udev race condition - some documentation changes - replaced `/sys/block/zram*` handling with `zramctl`, because I had occasional "Device is busy" error (looks like zram has to be configured in predefined order) - added `memoryPercent` and `algorithm` as restart triggers. I think, it was a bug that changing `memoryPercent` in configuration wasn't applied immediately. - removed a bind to .swap device. While it looks natural (when swap device goes off, so should zram device), it wasn't implemented properly. This caused problems with swapon/swapoff: ``` $ cat /proc/swaps Filename Type Size Used Priority /dev/zram0 partition 8166024 0 -2 /var/swapfile file 5119996 5120 1 $ sudo swapoff -a $ sudo swapon -a swapon: /dev/zram0: read swap header failed $ cat /proc/swaps Filename Type Size Used Priority /var/swapfile file 5119996 0 1 ```

danbst · 2019-01-17T14:04:51Z

@Mic92 addressed your issues. Also, added some release notes.

I'll merge in a week if nobody beats me

This creates a dependency cycle when used with boot.tmpOnTmpfs: basic.target <- tmp.mount <- swap.target <- zram-init-dev0 <- basic.target This same fix is done already for tmp.mount Fixes NixOS#47474

xaverdh · 2019-01-17T22:31:57Z

I can confirm, that the patch resolves the dependency cycle.

danbst requested a review from infinisil as a code owner December 27, 2018 22:22

GrahamcOfBorg added 6.topic: nixos 8.has: module (update) 10.rebuild-darwin: 0 10.rebuild-linux: 1-10 labels Dec 27, 2018

danbst changed the title ~~zramSwap: allow configure compression algorithm + cleanups~~ [WIP] zramSwap: allow configure compression algorithm + cleanups Dec 28, 2018

infinisil added the 2.status: work-in-progress label Dec 31, 2018

danbst force-pushed the zram-zstd branch from d8ff5ec to 7d0b691 Compare January 1, 2019 23:35

danbst changed the title ~~[WIP] zramSwap: allow configure compression algorithm + cleanups~~ zramSwap: allow configure compression algorithm + cleanups Jan 1, 2019

Mic92 mentioned this pull request Jan 14, 2019

Add netboot files NixOS/nixos-channel-scripts#21

Closed

Mic92 removed the 2.status: work-in-progress label Jan 14, 2019

Mic92 reviewed Jan 14, 2019

View reviewed changes

danbst force-pushed the zram-zstd branch from 33cf4fc to 862f2fc Compare January 17, 2019 13:52

GrahamcOfBorg added 8.has: changelog 8.has: documentation labels Jan 17, 2019

danbst force-pushed the zram-zstd branch from 862f2fc to 8d8a721 Compare January 17, 2019 13:59

danbst mentioned this pull request Jan 17, 2019

Unit ordering cycle #47474

Closed

zramSwap: remove basic.target for zram devices

34a764c

This creates a dependency cycle when used with boot.tmpOnTmpfs: basic.target <- tmp.mount <- swap.target <- zram-init-dev0 <- basic.target This same fix is done already for tmp.mount Fixes NixOS#47474

danbst merged commit ab31b13 into NixOS:master Jan 23, 2019

danbst deleted the zram-zstd branch January 23, 2019 07:31

bachp mentioned this pull request Feb 7, 2019

Reverts kernel bump + patch #55092

Merged

zowoq mentioned this pull request May 16, 2020

nixos/zram: make zstd the default #87917

Merged

jonringer mentioned this pull request Apr 16, 2024

[NixOS] Update zramSwap algorithm description #304638

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zramSwap: allow configure compression algorithm + cleanups #52991

zramSwap: allow configure compression algorithm + cleanups #52991

danbst commented Dec 27, 2018

worldofpeace commented Dec 27, 2018

danbst commented Dec 28, 2018

worldofpeace commented Dec 28, 2018

danbst commented Dec 28, 2018

danbst commented Jan 2, 2019

nixos-discourse commented Jan 2, 2019

Mic92 Jan 14, 2019 •

edited

danbst Jan 17, 2019

Mic92 Jan 14, 2019

Mic92 Jan 14, 2019

danbst commented Jan 17, 2019

xaverdh commented Jan 17, 2019

zramSwap: allow configure compression algorithm + cleanups #52991

zramSwap: allow configure compression algorithm + cleanups #52991

Conversation

danbst commented Dec 27, 2018

Motivation for this change

Things done

worldofpeace commented Dec 27, 2018

danbst commented Dec 28, 2018

worldofpeace commented Dec 28, 2018

danbst commented Dec 28, 2018

danbst commented Jan 2, 2019

nixos-discourse commented Jan 2, 2019

Mic92 Jan 14, 2019 • edited

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danbst commented Jan 17, 2019

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edited