4. Technical Description

A. SUMMARY

This case adds two new dataset-level properties, 'primarycache' and
'secondarycache',  to the existing zfs property infrastructure. These
properties control what is cached in the primary cache (ARC) and the
secondary cache (l2ARC).

B. USE CASES

One use case for not caching reads of user data is a large buffer cache
(part of the SGA) for Oracle.  If the admin wants to cache at the database
level, then it's pointless and wasteful to attempt to cache the same data
at the file system level as well.

One use case for not caching writes of user data is the log writer for
Oracle.  This data is only needed to redo transactions in the case of
a panic / unplanned shutdown.

We've also had users on zfs-discuss ask for the ability to control
what is cached.  Two such examples are noted here:
http://www.opensolaris.org/jive/thread.jspa?threadID=52159&tstart=0
http://www.opensolaris.org/jive/thread.jspa?messageID=202791&#202791

One reason we need to separate the control of the primary cache
from the secondary cache is to allow customers the ability to charge
for use of the l2ARC.

C. PROPOSED SOLUTION

The introduction of the 'primarycache' property will allow administrators
the ability to control what is cached in the ARC.  The possible choices
are: all, none, and metadata.  If 'all' is chosen, then both user data and
metadata are cached.  If 'none' is chosen, then neither user data nor
metadata is cached.  If 'metadata' is chosen, then only metadata is
cached.  The default is 'all'.

The introduction of the 'secondarycache' property will allow administrators
the ability to control what is cached in the l2ARC.  The possible choices
are the exact same as 'primarycache'.  The default is also 'all'.

The syntax for setting these zfs properties is as follows:

  # zfs set primarycache=metadata <pool/fs>
  # zfs create -o primarycache=metadata <pool/fs>

  # zfs set secondarycache=none <pool/fs>
  # zfs create -o secondarycache=none <pool/fs>

D. MANPAGE DIFFS

The following text will be added under the "Properties" section in zfs(1M):

  primarycache=all | none | metadata

      Controls what is cached in the primary cache (ARC).  If set to
      "all", then both user data and metadata is cached.  If set to
      "none", then neither user data nor metadata is cached.  If set to
      "metadata", then only metadata is cached.  The default behavior is
      "all".

  secondarycache=all | none | metadata

      Controls what is cached in the secondary cache (l2ARC).  If set
      to "all", then both user data and metadata is cached.  If set to
      "none", then neither user data nor metadata is cached.  If set to
      "metadata", then only metadata is cached.  The default behavior is
      "all".

Also, 'primarycache' and 'secondarycache' will be listed under allowable
permissions for 'zfs allow' in zfs(1M).