It's probably not efficient to have a temporary ZFS pool on top of a RAM disk.
After all the ZFS ARC and L2ARC are very efficient intermediate caches.
Anyway, it's possible to define a temporary ZFS pool on top of a RAM disk.
Perhaps (still have to check) I could disable ARC and L2ARC.
It's also possible that this ZFS pool be encrypted.
At this point, perhaps I could have a case to justify this idea.
But I'm not recommending nor advising anything.
For now, I'm just exploring technologies.
Assume I already have a RAM disk:
# ramdiskadm
Block Device Size Removable
/dev/ramdisk/c-01 268435456 Yes
On top of this block device I can establish the temporary ZFS pool.
And I want its root dataset to be encrypted right from the start.
# zpool create -R /c-01 -O encryption=on c-01 /dev/ramdisk/c-01
Enter passphrase for 'c-01':
Enter again:
After entering twice the passphrase the temporary ZFS pool will be created.
By default, a passphrase is prompted for encryption.
There's a space overhead for ZFS metadata.
# zpool list c-01
NAME SIZE ALLOC FREE CAP DEDUP HEALTH ALTROOT
c-01 250M 100K 250M 0% 1.00x ONLINE /c-01
# zpool status c-01
pool: c-01
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
c-01 ONLINE 0 0 0
/dev/ramdisk/c-01 ONLINE 0 0 0
The -R option automatically sets the altroot and cachefile properties.
# zpool get all c-01
NAME PROPERTY VALUE SOURCE
c-01 allocated 100K -
c-01 altroot /c-01 local
c-01 autoexpand off default
c-01 autoreplace off default
c-01 bootfs - default
c-01 cachefile none local
c-01 capacity 0% -
c-01 dedupditto 0 default
c-01 dedupratio 1.00x -
c-01 delegation on default
c-01 failmode wait default
c-01 free 250M -
c-01 guid 14233173042207590481 -
c-01 health ONLINE -
c-01 listshares off default
c-01 listsnapshots off default
c-01 readonly off -
c-01 size 250M -
c-01 version 34 default
The -O option sets the defaults for encryption.
Also note the default mountpoint for the associated root dataset.
# zfs get all c-01
NAME PROPERTY VALUE SOURCE
c-01 aclinherit restricted default
c-01 aclmode discard default
c-01 atime on default
c-01 available 218M -
c-01 canmount on default
c-01 casesensitivity mixed -
c-01 checksum sha256-mac local
c-01 compression off default
c-01 compressratio 1.00x -
c-01 copies 1 default
c-01 creation Thu Jul 10 11:20 2014 -
c-01 dedup off default
c-01 devices on default
c-01 encryption on local
c-01 exec on default
c-01 keychangedate Thu Jul 10 11:20 2014 local
c-01 keysource passphrase,prompt local
c-01 keystatus available -
c-01 logbias latency default
c-01 mlslabel none -
c-01 mounted yes -
c-01 mountpoint /c-01 local
c-01 multilevel off -
c-01 nbmand off default
c-01 normalization none -
c-01 primarycache all default
c-01 quota none default
c-01 readonly off default
c-01 recordsize 128K default
c-01 referenced 33K -
c-01 refquota none default
c-01 refreservation none default
c-01 rekeydate Thu Jul 10 11:20 2014 local
c-01 reservation none default
c-01 rstchown on default
c-01 secondarycache all default
c-01 setuid on default
c-01 shadow none -
c-01 share.* ... local
c-01 snapdir hidden default
c-01 sync standard default
c-01 type filesystem -
c-01 used 100K -
c-01 usedbychildren 67.5K -
c-01 usedbydataset 33K -
c-01 usedbyrefreservation 0 -
c-01 usedbysnapshots 0 -
c-01 utf8only off -
c-01 version 6 -
c-01 vscan off default
c-01 xattr on default
c-01 zoned off default
As I said earlier ZFS provides an intelligent and layered caching architecture comprised by the ARC (Adaptive Replacement Cache) and the L2ARC (Layer 2 ARC). Each ZFS dataset (file system, volume, etc...) has two properties for each type of cache, primarycache for ARC and secondarycache for L2ARC. Both properties provide a way to tell the ZFS engine how each cache should be used. Each property admits three possible values: all, metadata and none. As the backing ZFS pool device is a RAM disk I propose to evaluate the effect of setting both properties to none.
# zfs set primarycache=none c-01
# zfs set secondarycache=none c-01
# zfs get primarycache,secondarycache c-01
NAME PROPERTY VALUE SOURCE
c-01 primarycache none local
c-01 secondarycache none local
The way I've presented above, as I presumed since beginning, this experiment doesn't seem to present any advantage when the system is mostly idle or yet not heavily loaded. But it could be interesting to develop several scenarios in order to provide a better overall evaluation.
