ZFS ZFS on an md(4) device?

[Snowe]

Hi,

I've been experimenting with formatting md(4) memory disks with ZFS, the main idea being that I could use compression for more space over the usual UFS md.

I'm just wondering if there are any caveats with this, is it a bad idea?

So for example with:
mdconfig -s 4g -u md1
zpool create -o cachefile=none -m /ztmp -O compression=lz4 ztmp md1

With WRKDIRPREFIX=/ztmp in make.conf doing a make extract on lang/ghc uses 171MB.

If CPU is more important than space, zle compression gives 383MB usage.

UFS uses 529MB with the default blocksize.

I notice tmpfs uses almost the same as UFS but more available (I guess it's not reserving 8%).

 

[ondra_knezour]

The mdconfig(8) utility have the -o compress option, however I was not able to find any useful info about algorithms used etc. Maybe digging into code will say?

There also was experimental GEOM compression layer, so some research in this direction may reveal something useful for your scenario.

 

[Snowe]

Hi,
geom md list seems to indicate that -o compress does not operate on swap backed devices (but can on malloc backed devices).