ZRAM

WetGeek

I know what ZRAM does, but not a lot of details about how it works. I'm wondering whether its size, as reported in a partition manager, is its actual size on the disk, and how that relates to the amount of uncompressed data it can hold. To illustrate my question, here's an image of my laptop disk:

When I last installed KDE Plasma on this device, the installer created a ZRAM partition of approxiimately 8 GiB capacity. Does that mean that it occupies 8 GB on the SSD, or that it can only hold 8 GB of data after compressing it?

What this question is leading up to: I'm trying to figure out whether my current swap partition is redundant, with regard to allowing this system to hibernate. (The SSD in this laptop is 500 GB, which is excessive for my needs, but that's what was in it when I bought it.) The amount of RAM in this machine is 16 GB.

So, normally a 16 GB swap partition would be plenty big enough to allow the machine to hibernate, but in the extremely unlikely case that some of that swap space might already be in use at the time it was told to hibernate, and having far more room on that SSD that I'd ever need anyway, I asssigned the swap partition 24 GiB.

So, back to the question of compression for the ZDISK. Let's say that I had a whole lot of applications open, and my 16 GB of RAM was mostly in use, would the machine be able to hibernate with just that 8 GB of ZRAM available? Or would the laptop simply refuse to hibernate, as is the case with a too-small swap partition?

And a related question: What determined that the ZRAM partition in this machine would be 8 GB in size when I installed Solus? Is that a constant for every installation? Is it based on 1/2 the size of RAM in the machine? If it's configurable at install time, could the installer simply set it to the size of RAM in the system at install time?

If that last answer is true, then all machines could be able to hibernate without needing a separate swap partition to be created that's large enough to hold the contents of RAM. Just wondering.

(The advantages of a device's being able to hibernate are many, but I won't talk about them here. I've already done that in a different thread. This question is just about the relation of ZRAM capacity to its size on disk.)

ReillyBrogan

zram exists only in system memory and is never persisted to disk. An 8GB zram device means that the swap device can hold up to 8GB of compressed memory (but in practice it's unlikely to reach this point). Because zram compresses memory that 8GB of swap will actually occupy much less than 8GB of "real" memory, which is where the memory savings come from. The zram device is considered a swap device from the perspective of the kernel, so if you have a swap partition as well then you actually have 2 swap devices setup.

Zram shouldn't be an issue if you use hibernation since Linux will write the zram memory to the swap partition when it goes to hibernate.

The default memory size for the zram device is configured by zram-generator. The Solus configuration for this is installed to /usr/lib/systemd/zram-generator.conf which you can take a look at if you want. The default size comes from this line: zram-size = min(ram, 8192), which states that it will size it to how much system memory there is up to a maximum of 8GB. If you wanted to override this configuration you could copy this file to /etc/systemd/zram-generator.conf and modify it there.

WetGeek

ReillyBrogan zram exists only in system memory and is never persisted to disk.

Thanks for your very informative reply. The bottom line for me is that I should just continue doing what I do now.

There's only one question I still have. If we should consider that ZRAM is of no practical use to us, based on our hardware configuration, is there a simple way to avoid installing it in the first place? Or would deleting it in the partition manager when we create the real swap partition be a good idea? I assume there's no harm in that?

It's obviously not a big issue, but it's also not a big job to nuke it while I'm creating a swap partition.

ReillyBrogan

WetGeek Why is zram of "no practical use"? Zram is usually preferable to physical swap since compressing and decompressing pages in memory is much faster than writing or reading those pages from disc. Generally zram is either of no benefit (when the user has enough memory not to need it) or a benefit to the user, it is never a detriment which is why it's enabled by default by the installer. More advanced users who are sure they don't need it or would rather use zswap are more than welcome to disable it.

WetGeek

ReillyBrogan Why is zram of "no practical use"?

Beleive me, Reilly, I'm not trying to be argumentative, but isn't the purpose of swap somewhere you can put what's in memory if you memory is filled up and there's no more room? If RAM is filled up, what's the sense of swapping it to RAM? Just the compression? I can understand swapping to an SSD, but swapping RAM to RAM really puzzles me.

I'm really trying to understand this.

ReillyBrogan

WetGeek When you compress memory it takes up less space. So by swapping memory to the zram device you are causing it be compressed, since it's now compressed it takes us less space in memory and as a result you have "more" memory. This is why zram is helpful on devices that don't have a lot of memory since it makes the memory that they do have stretch much further than it would otherwise before they run out of it (at which point they will need to swap to disk or something will need to be closed if they don't have swap). Writing to and reading from this compressed memory space is much slower than writing to and reading from uncompressed memory yes, but still much much faster than reading from or writing to a disk (even a really fast SSD).

Or, to put it in numbers, let's say that a device has 8GB of memory. With Zram that device may be able to go up to 12GB of memory used before it needs to start swapping to a disk. That makes a big difference for users who may now be able to several more applications open before they start noticing their devices starting to slow down due to swapping.