On Sat, Jun 6, 2020 at 10:00 AM Richard W.M. Jones
Compression is intrinsic to just the /dev/zram device. The swap code
doesn't share pages between swap devices. The higher priority device
is favored first until full. Once full, pages don't go through the
zram module, thus are not compressed, on their way to the
So yeah, the swap-on-disk scenario might be better suited to a
generator that could use zswap instead, which uses an existing swap
partition and adds a write back cache (zpool) rather than a separate
device. I'm pretty sure (not 100%) that cached page are decompressed
on their way to the swap device. Also, the zpool memory cache is
preallocated, unlike zram devices.
(I am not going to envy any who decide to implement zswap on a system
with ZFS. Wait wait wait, which zpool are you talking about?!)
So, I am testing ZRAM right now (as per your advice in another thread). All well
so far, however reading this makes me think I gonna stumble upon a point where
ZSRAM will be a better fit.
You see, the idea of ZRAM and ZSWAP is improving low-memory situation. This is
especially relevant for small amount of RAM, like your Raspberry example.
In such situation if you, for example, open a lot of tabs in a browser, you may
easily get to a point where even ZRAM is exhausted. Now, had you additionally a
SWAP device, it would be no problem, the data would simply spill over to SWAP.
Yes, SWAP is slow (well, it is on HDD at least). But consider this: in this
workload , you most likely not gonna touch older of browser tabs for quite some
time, so the slowness won't hurt you.
My point is that we still need disk swap. And if we have disk swap, we'd want to
move into SWAP the most unused memory pages. Which is how it works with
ZSWAP. But not how it works with ZRAM (in which case, as you noted, once it's
full, all new data would simply go past ZRAM into disk SWAP)
Now, I love the idea of using either ZRAM or ZSWAP. But to consider which one of
them do we want, I think we would need to discuss first: do we really want to get
rid of disk swap? Hibernation being discussed somewhere in this thread is another
point. I personally don't like idea of removing disk swap.