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    # Simple Analysis of btrfs zstd compression level ## Workstation root fs disk space savings ### tl;dr; For an installed root directory of Fedora 32, zstd:1 yields a 40% storage savings as compared to uncompressed ext4. zstd:9 yields a 43% storage savings. ### Test Steps 1. Obtain an “installed.dir” by 1. Prep the image 1. truncate -s 64G installed.raw 2. Booting the live cd (in the case F32) in a qemu-kvm 1. qemu-kvm -drive file=installed.raw -cdrom $ISO -m 1G 3. Installing it in the graphical installer 1. I used regular partitions, and did not create a new one for /home 4. Shutting down the qemu instance3105440 5. Mount it to the directory 1. losetup -Pf installed.raw 2. mkdir -p installed.dir 3. mount /dev/loop1p2 installed.dir 2. Run the script below ``` #!/bin/sh set -e SIZE=16G if [ $EUID -ne 0 ] ; then echo "Needs to be root" >&2 exit 2 fi for compress_level in 1 3 9 ; do raw=zstd${compress_level}.raw truncate -s $SIZE $raw mkfs.btrfs -f $raw dir=zstd${compress_level}.dir mkdir -p $dir mount -o compress=zstd:${compress_level} $raw $dir perf record -g -o perf${compress_level}.data bash -c "cp -r installed.dir/* $dir ; sync $dir" done ``` ### Results zstd:1 - 40% savings zstd:3 - 41% savings zstd:9 - 43% savings This is the result from df. Ignore Use% because the installed directory is a different size. ``` Filesystem 1K-blocks Used Available Use% Mounted on /dev/loop1p3 61665068 5458980 53043976 10% /scratch/installed.dir /dev/loop0 16777216 3257192 13356936 20% /scratch/zstd1.dir /dev/loop2 16777216 3197668 13415468 20% /scratch/zstd3.dir /dev/loop3 16777216 3105440 13502704 19% /scratch/zstd9.dir ``` ## /dev/urandom copy cpu cost **note: this test is a lot less scientific than the other one, because it’ll be CPU dependent and I had other stuff going on (chrome was open, etc). It’s likely still a reasonable proxy for CPU impact though.** ### tl;dr; zstd:1 added 9% and zstd:9 added 15% of system time to the baseline. ### Test Steps 1. Get a 2GB blah file that will compress very poorly 1. dd if=/dev/urandom of=blah bs=256K count=8192 2. Run the script ``` #!/bin/sh set -e SIZE=16G if [ $EUID -ne 0 ] ; then echo "Needs to be root" >&2 exit 2 fi for compress_level in 0 1 3 9 ; do raw=zstd${compress_level}.raw truncate -s $SIZE $raw #perf record -g -o perf${compress_level}.data bash -c #perf record -g -o perf${compress_level}.data bash -c mkfs.btrfs -f $raw 2>/dev/null >/dev/null dir=zstd${compress_level}.dir mkdir -p $dir if [ $compress_level -eq 0 ] ; then mount $raw $dir else mount -o compress=zstd:${compress_level} $raw $dir fi echo Compression Level ${compress_level} time bash -c \ "dd if=blah of=$dir/blah ; sync $dir" done ``` ### Results ``` Compression Level 0 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 13.3657 s, 161 MB/s real 0m13.386s user 0m2.891s sys 0m9.486s Compression Level 1 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 14.8094 s, 145 MB/s real 0m14.913s user 0m3.313s sys 0m10.380s Compression Level 3 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 15.1291 s, 142 MB/s real 0m15.259s user 0m3.261s sys 0m10.720s Compression Level 9 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 15.4792 s, 139 MB/s real 0m15.499s user 0m3.442s sys 0m10.913s ``` ## /dev/zero cpu copy test ### tl;dr; Easily compressed data is a lot more expensive at higher levels. zstd:1 added 16%, zstd:9 added 82% ### Test Steps Same as above, but swap /dev/urandom for /dev/zero ### Results ``` Compression Level 0 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 14.1791 s, 151 MB/s real 0m14.196s user 0m2.971s sys 0m9.909s Compression Level 1 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 16.4391 s, 131 MB/s real 0m16.536s user 0m3.403s sys 0m11.511s Compression Level 3 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 14.9807 s, 143 MB/s real 0m15.094s user 0m3.398s sys 0m10.833s Compression Level 9 4194304+0 records in 4194304+0 records out 2147483648 bytes (2.1 GB, 2.0 GiB) copied, 18.0451 s, 119 MB/s real 0m18.066s user 0m3.758s sys 0m12.173s ```

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