# SSD for L2ARC and ZIL priority tuning



## belon_cfy (May 22, 2013)

Hi,

My ZFS storage is using single SSD as L2ARC and ZIL. May I know how can I raise the ZIL write priority to reduce interruption from L2ARC read? I found that the write latency significantly increases when the L2ARC read rate is high. 

SSD powered by SandForce SF2000 series is the best choice for such a situation because the write latency won't be affected by read activity; of course the read rate will be slower than the rest of the competitors.

I have already tuned the following parameters for reducing L2ARC write, what else I can do beside this?


```
vfs.zfs.l2arc_noprefetch=0
vfs.zfs.l2arc_feed_min_ms=1000
```


----------



## belon_cfy (May 23, 2013)

Today we bought another Sandforce based SSD and it performs quite well as a l2arc and zil. The latency is much lower than other non-Sandforce-based SSDs.

I will stick with Sandforce-based SSD for better latency.


----------



## phoenix (May 25, 2013)

Try setting 
	
	



```
vfs.zfs.l2arc_norw=0
```
 in /etc/sysctl.conf

The default prevents writes to L2ARC devices if there ate pending writes. Might not be applicable to shared L2/log devices, but it really helps with large L2 devices and large pools.


----------



## belon_cfy (May 27, 2013)

phoenix said:
			
		

> Try setting
> 
> 
> 
> ...



Thanks for your recommendation, it does help a bit but it still does not completely solve the problem.


----------



## ta0kira (Jun 7, 2013)

phoenix said:
			
		

> Try setting
> 
> 
> 
> ...


Does this need to be in /boot/loader.conf instead, so it's set before the pools are mounted?

Kevin Barry


----------

