Feature: Use lz4 compression for metadata where available

[DeHackEd]

Currently metadata compression (if not disabled by the zfs_mdcomp_disable parameter) is explicitly set to LZJB. If the LZ4 feature flag is active it probably makes sense to switch to using LZ4 in its place.

See zfs/modules/zfs/dmu.c function dmu_write_policy

[behlendorf]

It would be useful to measure if there are really any space saving to be had for metadata when using lz4 over lzjb. If the savings are minimal it may be wiser to leave it stored as lzjb which is understood by virtually all of the existing implementations.

[DeHackEd]

I thought about that, especially when 4k disks make the benefits shrink. That's why I said only if the state is already active. By that time it's too late for any other implementations to be able to read it since the state is irreversible and has no compatibility support. Might as well enjoy the improved decompression speed.

I also thought about potential alternative implementations such as inheriting the compression setting from the filesystem properties, but somehow gzip-9 compressed metadata doesn't sit well with me. :)

[ryao]

@DeHackEd Nice catch. This explains why lzjb appeared to be in use on my pool where I was doing lz4. With that said, this issue also affects Illumos. It probably would make sense to discuss it with them on their mailing list.

@behlendorf The Illumos wiki might be a good reference here:

A rough comparative analysis of LZ4 performance vs. LZJB:

* Approximately 50% faster compression when operating on compressible data.
* Approximately 80% faster on decompression.
* Over three times faster on compression of incompressible data.
* Higher compression ratio (up to 10% on the larger block sizes).
* Performance on modern CPUs often exceeds 500 MB/s on compression and over 1.5 GB/s on decompression and incompressible data (per single CPU core).

http://wiki.illumos.org/display/illumos/LZ4+Compression

My understanding of lz4 versus lzjb is that lzjb would not have been invented had lz4 been available when ZFS were first made. lz4 is superior to lzjb by virtually all metrics.

[aikudinov]

illumos feature request: https://www.illumos.org/issues/3756

[maci0]

+1

[thegreatgazoo]

Looks like it was already committed on Illumos:
illumos/illumos-gate@b8289d2