Let's talk about zram.
I just got around to figuring out what zram is. I see it's automatically
set up when Anaconda sets up btrfs.
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.
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:
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.
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.
If this is necessary to make btrfs work or get reasonable performance
from it. That is a strike against btrfs.
From this I conclude that to run btrfs on these machines and get the
performance we're used to I have to up the minimum ram load to 12GB. Ram
modules have a non-zero cost. Given the above this cost must be assigned
to btrfs and are born by Fedora users.
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.
Have a Great Day!
Pat (tablepc)
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