On 7/20/20 7:23 AM, pmkellly@xxxxxxxxxxxx wrote:
I just got around to figuring out what zram is. I see it's automatically set up when Anaconda sets up btrfs.
It shouldn't have anything to do with btrfs. It's one of the changes to have it turned on by default. Look in the devel list archives for some huge threads on this.
First a bit of background all the PCs (Fedora WS) I maintain are bought with lots of ram 8GB is the min. I don't think I've ever seen swap move off zero and no one has reported any sort of slowdowns. Yet we do lots of memory intensive things. Of course all this is with ext4.
That is surprising. Usually at least something ends up in the swap.
Now I see on my test machine (btrfs) that about 24% (2GB) of the 8GB of memory (according to monitor) (4GB according to disks) is dedicated to zram0. I imagine the difference in size reported to due to the compression used by zram. However there are two issues:
Careful what you're using to measure with. The only accurate measurement is "zramctl". That will tell you exactly how much RAM the zram device is using.
First I can't think of any good reason why I should need or want to have any of my ram dedicated to swapping or other things that speed up btrfs. We've been very happy with swap being on disk since it's never been seen to move off zero.
You really need to read more about zram. There is no dedicated space for it. It only uses RAM as swap is needed. And it uses less RAM than the pages that are getting swapped out, so there's a net reduction in RAM usage without hitting the disk.
Second, a few years back, I looked over some of the compression algorithms. The probability of loss or corruption was low, but non-zero. I'm sure they have improved, but I'll bet those probabilities are still non-zero. I was taught back in my early years at school that unnecessary risk is foolish risk. So I've always avoided using compression where ever I had an option.
These compression algorithms have been tested very hard and are used everywhere. I've never seen mention of any corruption issues.
I'm hoping someone can tell me I'm wrong and why, or that there's a way to opt-out of zram without a hit to performance when using btrfs.
If you really want to disable zram (there's no reason to), I believe the simplest method is "touch /etc/systemd/zram-generator.conf".
And it's nothing to do with btrfs. _______________________________________________ test mailing list -- test@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to test-leave@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/test@xxxxxxxxxxxxxxxxxxxxxxx
