ZFS Disabling Metadata Compression

Disabling Metadata Compression

Caution: This tuning needs to be researched as it’s now apparent that the tunable applies only to indirect blocks leaving a lot of metadata compressed anyway.

With ZFS, compression of data blocks is under the control of the file system administrator and can be turned on or off by using the command "zfs set compression …".

On the other hand, ZFS internal metadata is always compressed on disk, by default. For metadata intensive loads, this default is expected to gain some amount of space (a few percentages) at the expense of a little extra CPU computation. However, a bigger motivation exists to have metadata compression on. For directories that grow to millions of objects then shrink to just a few, metadata compression saves large amounts of space (>>10X).

In general, metadata compression can be left as is. If your workload is CPU intensive (say > 80% load) and kernel profiling shows medata compression is a significant contributor and we are not expected to create and shrink huge directories, then disabling metadata compression can be attempted with the goal of providing more CPU to handle the workload.

Solaris 10 11/06 and Solaris Nevada (snv_52) Releases

Set dynamically:

echo zfs_mdcomp_disable/W0t1 | mdb -kw

Revert to default:

echo zfs_mdcomp_disable/W0t0 | mdb -kw

Set the following parameter in the /etc/system file:

set zfs:zfs_mdcomp_disable = 1

Earlier Solaris Releases

Not tunable.

RFEs

* 6391873 metadata compression should be turned back on (Integrated in NEVADA snv_36)

http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6391873

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