Add docs for using SquashFS on targets w/o zstd

conda_pack/formats.py

@@ -402,13 +402,18 @@ def mksquashfs_from_staging(self):
             cmd += ["-comp", comp_algo_str]
         else:
             comp_level = int(self.compress_level / 8 * 20)
-            comp_algo_str = "zstd level {}".format(comp_level)
+            comp_algo_str = "zstd (level {})".format(comp_level)
             # 256KB block size instead of the default 128KB for slightly smaller archive sizes
             cmd += ["-comp", "zstd", "-Xcompression-level", str(comp_level), "-b", str(256*1024)]
 
         if self.verbose:
-            print("Running mksquashfs (processors: {}, compression: {})".format(
-                self.n_threads, comp_algo_str))
+            s = "Running mksquashfs with {} compression (processors: {}).".format(
+                comp_algo_str, self.n_threads)
+            if self.compress_level != 9:
+                s += "\nWill require kernel>=4.14 or squashfuse>=0.1.101 (compiled with zstd) " \
+                     "for mounting.\nTo support older systems, compress with " \
+                     "`xz` (--compress-level 9) instead."
+            print(s)
         else:
             cmd.append("-no-progress")
         subprocess.check_call(cmd)

docs/source/squashfs.rst

@@ -4,8 +4,7 @@ SquashFS
 ``conda-pack`` can package environments into
 `SquashFS <https://en.wikipedia.org/wiki/SquashFS>`_, a compressed, read-only Linux filesystem.
 These filesystems can then be mounted directly, without decompressing them first.
-This allows benefiting from compressed storage, without the decompression step necessary for the
-``zip`` and ``tar.gz`` formats.
+This allows using packed environments immediately and without consuming disk space.
 
 Packing
 -------
@@ -73,7 +72,13 @@ Compression options
 -------------------
 
 Compression can be specified through ``--compress-level``.
-Default is level 4.
+Default is level 4, which will use ``zstd`` compression.
+
+When selecting a compression option for packing, keep in mind the kernel or ``squashfuse`` version on the target system.
+For older systems, make sure that SquashFS or ``squashfuse`` support ``zstd`` compression
+(Linux kernel version ``>=4.14`` or ``squashfuse >= 0.1.101``).
+If ``zstd`` isn't supported on the target, you can always compress with ``xz`` instead.
+
 
 - 0: no compression
 - 1-8: ``zstd`` with increasing compression level
@@ -87,10 +92,10 @@ Sometimes the unpacked environment needs to be writeable on the target machine,
 more packages.
 A good way to do this is to use `Union mounting <https://en.wikipedia.org/wiki/Union_mount>`_ to
 add a writeable layer on top of the read-only SquashFS.
-
 On Linux the most used option is `OverlayFS <https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html>`_.
 
-To show how to set this up, we create three layers:
+To set this up, we create three layers:
+
 1. The SquashFS-packed conda env as a read-only lower layer
 2. A writeable working directory, necessary for OverlayFS
 3. A writeable upper directory, where all new and changed files will go