re-enable zramSwap on all devices

this is critical on moby, though even with this swap, we run out of CMA (videoram) instead -- just later

hosts/by-name/servo/fs.nix

@@ -59,11 +59,6 @@
     }
   ];
 
-  # in-memory compressed RAM (seems to be dynamically sized)
-  # zramSwap = {
-  #   enable = true;
-  # };
-
   # btrfs doesn't easily support swapfiles
   # swapDevices = [
   #   { device = "/nix/persist/swapfile"; size = 4096; }

hosts/common/fs.nix

@@ -60,6 +60,28 @@ in
   # some services which use private directories error if the parent (/var/lib/private) isn't 700.
   sane.fs."/var/lib/private".dir.acl.mode = "0700";
 
+  # in-memory compressed RAM
+  # defaults to compressing at most 50% size of RAM
+  # claimed compression ratio is about 2:1
+  # - but on moby w/ zstd default i see 4-7:1  (ratio lowers as it fills)
+  # note that idle overhead is about 0.05% of capacity (e.g. 2B per 4kB page)
+  # docs: <https://www.kernel.org/doc/Documentation/blockdev/zram.txt>
+  #
+  # to query effectiveness:
+  # `cat /sys/block/zram0/mm_stat`. whitespace separated fields:
+  # - *orig_data_size*  (bytes)
+  # - *compr_data_size* (bytes)
+  # - mem_used_total    (bytes)
+  # - mem_limit         (bytes)
+  # - mem_used_max      (bytes)
+  # - *same_pages*  (pages which are e.g. all zeros (consumes no additional mem))
+  # - *pages_compacted*  (pages which have been freed thanks to compression)
+  # - huge_pages  (incompressible)
+  #
+  # see also:
+  # - `man zramctl`
+  zramSwap.enable = true;
+
   # fileSystems."/mnt/servo-nfs" = {
   #   device = "servo-hn:/";
   #   noCheck = true;