How to turn on compression for a single file on btrfs

I did a small benchmark. It only tests writes though.

Test data is a Linux kernel source tree (linux-3.8), already unpacked into memory (/dev/shm/ tmpfs), so there should be as little influence as possible from the data source. I used compressible data for this test since compression with non-compressible files is nonsense regardless of encryption.

Using btrfs filesystem on a 4GiB LVM volume, on LUKS [aes, xts-plain, sha256], on RAID-5 over 3 disks with 64kb chunksize. CPU is a Intel E8400 2x3Ghz without AES-NI. Kernel is 3.8.2 x86_64.

The script:

#!/bin/bash

PARTITION="/dev/lvm/btrfs"
MOUNTPOINT="/mnt/btrfs"

umount "$MOUNTPOINT" >& /dev/null

for method in no lzo zlib
do
    for iter in {1..3}
    do
        echo Prepare compress="$method", iter "$iter"
        mkfs.btrfs "$PARTITION" >& /dev/null
        mount -o compress="$method",compress-force="$method" "$PARTITION" "$MOUNTPOINT"
        sync
        time (cp -a /dev/shm/linux-3.8 "$MOUNTPOINT"/linux-3.8 ; umount "$MOUNTPOINT")
        echo Done compress="$method", iter "$iter"
    done
done

So in each iteration, it makes a fresh filesystem, and measures the time it takes to copy the linux kernel source from memory and umount. So it's a pure write-test, zero reads.

The results:

Prepare compress=no, iter 1

real 0m12.790s
user 0m0.127s
sys 0m2.033s
Done compress=no, iter 1
Prepare compress=no, iter 2

real 0m15.314s
user 0m0.132s
sys 0m2.027s
Done compress=no, iter 2
Prepare compress=no, iter 3

real 0m14.764s
user 0m0.130s
sys 0m2.039s
Done compress=no, iter 3
Prepare compress=lzo, iter 1

real 0m11.611s
user 0m0.146s
sys 0m1.890s
Done compress=lzo, iter 1
Prepare compress=lzo, iter 2

real 0m11.764s
user 0m0.127s
sys 0m1.928s
Done compress=lzo, iter 2
Prepare compress=lzo, iter 3

real 0m12.065s
user 0m0.132s
sys 0m1.897s
Done compress=lzo, iter 3
Prepare compress=zlib, iter 1

real 0m16.492s
user 0m0.116s
sys 0m1.886s
Done compress=zlib, iter 1
Prepare compress=zlib, iter 2

real 0m16.937s
user 0m0.144s
sys 0m1.871s
Done compress=zlib, iter 2
Prepare compress=zlib, iter 3

real 0m15.954s
user 0m0.124s
sys 0m1.889s
Done compress=zlib, iter 3

With zlib it's a lot slower, with lzo a bit faster, and in general, not worth the bother (difference is too small for my taste, considering I used easy-to-compress data for this test).

I'd make a read test also but it's more complicated as you have to deal with caching.

It seems that only approach left is stacking a compressing filesystem on top of encrypting filesystem. Here are few options, but I do not have hands on experience with them:

• FuseCompress hosted on GitHub
• SquashFS, JFFS2, or some combination described on LWN

I doubt that you can select compression or encryption per file. Btrfs is supposed to offer that but reality is probably far from it. Just like it was supposed to have RAID5 years ago.