SIMPLE SOLUTIONS

COMPSIZE(8) - Linux man page online | Administration and privileged commands

Calculate compression ratio of a set of files on btrfs.

Chapter
2017-09-04
compsize(8) btrfs compsize(8)

NAME

compsize - calculate compression ratio of a set of files on btrfs

SYNOPSIS

compsize file-or-dir [ file-or-dir ... ]

DESCRIPTION

compsize takes a list of files on a btrfs filesystem (recursing directories) and measures used compression types and the effective compression ratio. As talking about compression ratio for a partial extent doesn't quite make any sense, every used extent is considered in its entirety. Every extent is also counted exactly once, even if it's reflinked multiple times. The program gives a report similar to: Processed 90319 files. Type Perc Disk Usage Uncompressed Referenced TOTAL 79% 1.4G 1.8G 1.9G none 100% 1.0G 1.0G 1.0G lzo 53% 446M 833M 843M The fields above are: Type compression algorithm Perc disk usage/uncompressed (compression ratio) Disk Usage blocks on the disk; this is what storing these files actually costs you (save for RAID considerations) Uncompressed uncompressed extents; what you would need without compression - includes deduplica‐ tion savings and pinned extent waste Referenced apparent file sizes (sans holes); this is what a traditional filesystem that sup‐ ports holes and efficient tail packing, or tar -S, would need to store these files Let's see this on an example: a file 128K big is stored as a single extent A which com‐ pressed to a single 4K page. It then receives a write of 32K at offset 32K, which also compressed to a single 4K page, stored as extent B. The file now appears as: +-------+-------+---------------+ extent A | used | waste | used | +-------+-------+---------------+ extent B | used | +-------+ The "waste" inside extent A can't be gotten rid until the whole extent is rewritten (for example by defrag). If compressed, the whole extent needs to be read every time that part of the file is being read, thus the "waste" is still required. In this case, we have: Disk Usage: 8KB, Uncompressed: 160K, Referenced: 128K.

OPTIONS

-b/--bytes Show raw byte counts rather than human-friendly sizes. -x/--one-file-system Skip files and directories on different file systems.

CAVEATS

Recently written files may show as not taking any space until they're actually allocated and compressed; this happens once they're synced or on natural writeout, typically on the order of 30 seconds. The ioctls used by this program require root.
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