{"id":68534,"date":"2022-11-11T09:00:41","date_gmt":"2022-11-11T09:00:41","guid":{"rendered":"https:\/\/www.cryptocabaret.com\/?p=68534"},"modified":"2022-11-11T09:00:41","modified_gmt":"2022-11-11T09:00:41","slug":"drop-swap-for-zram-on-linux","status":"publish","type":"post","link":"https:\/\/www.cryptocabaret.com\/?p=68534","title":{"rendered":"Drop swap for zram on Linux"},"content":{"rendered":"

Drop swap for zram on Linux<\/span>
\nDavid Both<\/a><\/span>
\nFri, 11\/11\/2022 – 03:00<\/span><\/p>\n

\n

I spend a lot of time playing (I mean working<\/em>) on my computers, and I’ve found a lot of interesting things. One that has most recently come to my attention is the zram0<\/code> device. I first noticed it when working on one of my Opensource.com articles several months ago. It showed up in the output from the lsblk<\/code> command:<\/p>\n

\n
# lsblk<\/span>
\nNAME \u00a0 \u00a0 \u00a0 \u00a0 \u00a0MAJ:MIN RM \u00a0 SIZE RO TYPE MOUNTPOINTS
\nsda \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 8<\/span>:0<\/span> \u00a0 \u00a00<\/span> 931.5G \u00a00<\/span> disk
\n\u251c\u2500sda1 \u00a0 \u00a0 \u00a0 \u00a0 \u00a08<\/span>:1<\/span> \u00a0 \u00a00<\/span> \u00a0 600M \u00a00<\/span> part
[<\/span>...]<\/span>
\nzram0 \u00a0 \u00a0 \u00a0 \u00a0 252<\/span>:0<\/span> \u00a0 \u00a00<\/span> \u00a0 \u00a0 8G \u00a00<\/span> disk [<\/span>SWAP]<\/span><\/div><\/div><\/pre>\n
It’s identified as swap space, which is what first piqued my curiosity, so I did some exploration. Zram was originally called “compcache,” which stands for “compressed cache.” It turns out that zram is a tool for creating an in-RAM compressed cache, specifically for use as swap space.<\/p>\n
But Why?<\/p>\n<\/p>\n
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When I began researching zram, all I found were a couple of basic articles about using zram for swap space. At first, this seemed a bit counterintuitive to me. After all, if you’re running out of RAM and you swap pages into a virtual drive in RAM, what’s gained?<\/p>\n
I then found the Fedora Project wiki page that proposed the use of Swap on zram<\/a>. The proposal says: “Swap is useful, except when it’s slow. zram is a RAM drive that uses compression. Create a swap-on-zram during start-up. And no longer use swap partitions by default.”<\/p>\n
The rest of the page is about details, benefits, side effects, and feedback.<\/p>\n
Zram for swap space on Linux<\/h2>\n
Using zram for swap space is intended to do the same thing as regular partition-based or file-based swap space. When memory pressure becomes too great, some of the least recently used data is moved to swap space. On average, it’s compressed to about 50% of its original size, and placed in zram space in RAM. This is much faster than storing those memory pages on a hard drive and frees up the RAM it was using for other use.<\/p>\n
Saving on swap<\/h2>\n
I tried to find revised recommendations for how much swap or zram swap to configure. This led me back to a reassessment of swap, and my previous article, What’s the right amount of swap space for a modern Linux system?<\/a> As far as I can tell from the most current documentation for RHEL and Fedora, the recommended amount of swap space has not changed. That documentation, however, ignores the use of zram.<\/p>\n
However, the tables in that previous article still provide a good starting point for swap space allocation when using older releases of Linux that don’t use zram or in cases where zram has been disabled.<\/p>\n
The documents I found for the Zram feature are inconsistent in terms of how zram is allocated with respect to RAM size, and the amount of space allocated to zram swap.<\/p>\n
Due to the lack of authoritative documentation, I performed some experiments to empirically determine the algorithm used to allocate zram swap. I used my own physical and virtual systems for this. The results are interesting and do not match any documentation I’ve so far found.<\/p>\n
The default size of zram is 8 GB on all systems large enough to support that, but it’s typically reduced significantly on hosts with small amounts of RAM. On one virtual machine (VM) I use for testing, with access to 4 GB of RAM, the zram virtual swap space is allocated to 3.8 GB. One old Dell I have contains 8 GB of RAM, and the zram is set to 7.6 GB. When RAM is reduced to 2 GB, Zram is reduced to 1.9 GB.<\/p>\n
All physical and virtual hosts I have with more than 8 GB of RAM show exactly 8 GB of zram. This includes my primary workstation with 64 GB of RAM and other hosts with 16 GB or 32 GB of RAM.<\/p>\n
Based on these few data points, I can draw the conclusion that the current default settings are for 8 GB of zram at most, and for zram to be 95% of RAM on hosts with 8 GB or less.<\/p>\n
I have read a number of articles that mention other sizes for zram swap, even up to 100% of RAM, but those all seem to be theoretical rather than reality.<\/p>\n
Your distribution may be different, but here are the actual zram swap allocations for Fedora and similar distributions:<\/p>\n