Re: Reasonably simple setup for 1TB HDD and 250GB M.2 NVMe SSD
Hi Piotr! Happy 2022!
Em [2021-12-08 qua 22:54:29+0000], piorunz escreveu:
> On 08/12/2021 19:35, Jorge P. de Morais Neto wrote:
>> - Why `compress-force' instead of simply `compress'?
>
> I've read very extensive discussion about that and came to conclusion
> that compress-force is better. It's checking every chunk of file for
> compressibility. "compress", on the other hand, checks only first
> sectors, then drops compression if no compressible data is detected.
> Imagine your qcow file, first 1 GB is not compressible, so "compress"
> option will drop compression of that file right away. But remaining 20GB
> are zeros because you haven't filled that yet. With compress-force, you
> compress these zeros to nothing. File takes 1GB of space. You don't have
> that on ext4, or btrfs "compress" only option.
Have you revised after kernel Linux 4.15? The btrfs(5) manpage says:
Since kernel 4.15, a set of heuristic algorithms have been
improved by using frequency sampling, repeated pattern
detection and Shannon entropy calculation to avoid that.
Therefore, it looks like after kernel Linux 4.15 the compress option (or
compress=ALG:LEVEL), instead of compress-force, became more interesting.
I am currently using compress-force=zstd:12 for the SSD and
compress=zstd:12 for both HDD (internal SATA and external USB3)¹.
Despite the strong compression level, performance is pretty good. Yet,
when the system settles, I intend to reduce compression level to 9 or 6
(as you earlier recommended). This should make performance even better,
while saving a lot of space because most data was compressed at
level 12.
And I may also change compress-force to compress, even for the SSD,
because I run kernel 5.10 which is later than 4.15. I may ask the
linux-btrfs mailing list first. I have subscribed to it. Are you there
too?
Regards
* Footnotes
¹ Both HDD have compress (rather than compress-force) because most of
their files are already compressed---pictures, videos, music, compressed
archives etc.
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