ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks

Daniel Kalchev daniel at digsys.bg
Thu Jan 6 14:13:25 UTC 2011

For pure storage, that is a place you send/store files, you don't really 
need the ZIL. You also need the L2ARC only if you read over and over 
again the same dataset, which is larger than the available ARC (ZFS 
cache memory). Both will not be significant for 'backup server' 
application, because it's very unlikely to do lots of SYNC I/O (where 
separate ZIL helps), or serve the same files back (where the L2ARC might 

You should also know that having large L2ARC requires that you also have 
larger ARC, because there are data pointers in the ARC that point to the 
L2ARC data. Someone will do good to the community to publish some 
reasonable estimates of the memory needs, so that people do not end up 
with large but unusable L2ARC setups.

It seems that the upcoming v28 ZFS will help greatly with the ZIL in the 
main pool..

You need to experiment with the L2ARC (this is safe with current v14 and 
v15 pools) to see if your usage will see benefit from it's use. 
Experimenting with ZIL currently requires that you recreate the pool. 
With the experimental v28 code things are much easier.

On 06.01.11 15:11, Damien Fleuriot wrote:
> I see, so no dedicated ZIL device in the end ?
> I could make a 15gb slice for the OS running UFS (I don't wanna risk
> losing the OS when manipulating ZFS, such as during upgrades), and a
> 25gb+ for L2ARC, depending on the disk.
> I can't afford a *dedicated* drive for the cache though, not enough room
> in the machine.

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