03-test-kernel: tweak squashfs compression

A larger block size was recommended by led@; gns@ seems to concur as the 512k value was borrowed from liveflash.eeepc profile (along with -noI). The other issue is with binary specific compressors: x86 was clearly assumed while the data for an educated guess are pretty handy. Please note that using filters incurs additional compression attempts for the utility to choose the best result.

03-test-kernel: tweak squashfs compression
178a700e · Michael Shigorin · 3fcbed33 · 178a700e
Commit 178a700e authored Jan 18, 2012 by Michael Shigorin
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03-test-kernel features.in/stage2/stage1/scripts.d/03-test-kernel +12 -2

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--- a/features.in/stage2/stage1/scripts.d/03-test-kernel
+++ b/features.in/stage2/stage1/scripts.d/03-test-kernel
 #!/bin/sh
 # check relevant kernel features availability

+GZ_OPTS="-comp gzip"
+XZ_OPTS="-comp xz -b 524288 -noI"
+
 # test for installer-required filesystems support
 for opt in CONFIG_SQUASHFS CONFIG_AUFS_FS; do
 	if grep -q "^$opt=[my]$" /boot/config-*; then
@@ -16,7 +19,14 @@ done
 # squashfs options: not really neccessary but better than none
 # NB: this config file should be carried over into install2
 if grep -q '^CONFIG_SQUASHFS_XZ=y$' /boot/config-*; then
-	echo "PACK_SQUASHFS_OPTS=-comp xz -Xbcj x86" > /.image/squashcfg.mk
+	# TODO: figure out if it's generally worth it even on x86:
+	#       if binaries account for less than ~70% of blocks,
+	#       the decompression filter overhead might hurt
+	# NB: there are arm, powerpc and some other filters too
+	if grep -q "^CONFIG_X86" /boot/config-*; then
+		XZ_OPTS="$XZ_OPTS -Xbcj x86"
+	fi
+	echo "PACK_SQUASHFS_OPTS=$XZ_OPTS" > /.image/squashcfg.mk
 else
-	echo "PACK_SQUASHFS_OPTS=-comp gzip" >> /.image/squashcfg.mk
+	echo "PACK_SQUASHFS_OPTS=$GZ_OPTS" > /.image/squashcfg.mk
 fi