Improve mksquashfs compression
| Affects | Status | Importance | Assigned to | Milestone | |
|---|---|---|---|---|---|
| Snapcraft |
New
|
Undecided
|
Unassigned | ||
Bug Description
When snapcraft calls `mksquashfs`, it doesn't specify any block size so it defaults to 131072 (128K). Tweaking this to 1M leads to better compression:
$ sudo mksquashfs /snap/core22/
$ sudo mksquashfs /snap/core22/
$ ll -h *.snap
-rw-r--r-- 1 root root 72M Oct 4 15:23 core22-b1M.snap
-rw-r--r-- 1 root root 74M Oct 4 15:22 core22.snap
We can observe similar savings with lxd snap:
$ sudo mksquashfs /snap/lxd/current/ lxd.snap -noappend -no-progress -comp xz -no-fragments -all-root -no-xattrs
$ sudo mksquashfs /snap/lxd/current/ lxd-b1M.snap -noappend -no-progress -comp xz -no-fragments -all-root -no-xattrs -b 1M
$ ll -h *.snap
-rw-r--r-- 1 root root 170M Oct 4 15:29 lxd-b1M.snap
-rw-r--r-- 1 root root 182M Oct 4 15:31 lxd.snap
Another tweak that might be worth it for snaps is to use the `branch/call/jump (BCJ) filter` feature of XZ:
$ sudo mksquashfs /snap/core22/
$ sudo mksquashfs /snap/lxd/current/ lxd-b1M-Xbcj.snap -noappend -no-progress -comp xz -no-fragments -all-root -no-xattrs -b 1M -Xbcj x86,arm,powerpc
$ ll -h *.snap
-rw-r--r-- 1 root root 72M Oct 4 15:23 core22-b1M.snap
-rw-r--r-- 1 root root 70M Oct 4 15:38 core22-
-rw-r--r-- 1 root root 74M Oct 4 15:22 core22.snap
-rw-r--r-- 1 root root 170M Oct 4 15:36 lxd-b1M.snap
-rw-r--r-- 1 root root 163M Oct 4 15:35 lxd-b1M-Xbcj.snap
-rw-r--r-- 1 root root 182M Oct 4 15:31 lxd.snap
Note: changing the block size provides almost no gain when using lzo (which doesn't support BCJ filters):
$ sudo mksquashfs /snap/core22/
$ sudo mksquashfs /snap/core22/
$ ll -h core22-lzo*.snap
-rw-r--r-- 1 root root 98M Oct 4 15:39 core22-lzo-b1M.snap
-rw-r--r-- 1 root root 99M Oct 4 15:39 core22-lzo.snap
| affects: | snapd → snapcraft |
| Callahan Kovacs (mr-cal) wrote | #1 |
| Simon Déziel (sdeziel) wrote | #2 |
@sergio-j-cazzolato - I think snapd is the right category.
Snapcraft doesn't directly call `mksquashfs`. Instead, it calls `snap pack` and snapd makes the call to `mksquashfs`.