zfs commands to underprovision cache SSDs

[danwood82]

Hi guys, more novice zfs questions I'm afraid

I took the plunge and ordered up the parts for my server build. I'm just trying to work out any last unknowns before I set it all up once the kit arrives.

One thing I'm a bit in the dark on - I'm planning on underprovisioning the ZIL/L2ARC SSDs, but I realise that having gotten reasonably familiar with napp-it on OmniOS, I'm still clueless about how to allocate less than an entire drive to a zpool or as a cache device.

Can this be done in the napp-it interface, or would I need to delve into the command line to do this?

(Additionally, I'm not certain exactly how to set up a RAID-10-alike zpool in napp-it. I've only been testing with three drives in the past though, so it might have just been hiding the option from me. Either way, I've not had any trouble digging up the right zpool command to do it through the command line if necessary... would be interested to know how to go about it inside napp-it though)

 

[dswartz]

For underprovisioning the l2arc ssd(s), use format command. you want to create a solaris slice taking up say 90% of the ssd, then add that slice (ending with s0) to the pool as cache. sorry i don't remember the exact commands...

 

[gea]

A Zil usage can grow up to about 10s of data/network traffic, what means it can go up to about 1 GB on a Gb network. I have not done any tests if there is really a difference if you partition a SSD or just use as is when used capacity is very low.

If you want to try, you can partition or slice (slices are parts of a Solaris partition) a disks. Easiest way:
- enable partition support in napp-it (menu disks - partitions - enable partition support)
- partition the ssd (menu disks - partitions - partition disks)
- add a partition to your pool as ZIL

Attention:
Do not partition a SSD to use it as L2ARC and/or Bootdisk and ZIL together.
A Zil must be ultra low latency or it is worthless so a ZIL must be a dedicated disk - best are SSDs like a Intel S3700 (min capacity 100 GB). With such write optimized disks, I would not expect a real advantage of underprovision a disk.

With L2Arc, underprovisioning affects write performance but performance relevant usage is read. This is the reasons that you usually do not partition/slice disks but just use the whole disks.