ZFS: How to enable cache and logs.

[Daniel Kalchev]

On 12.05.11 05:26, Danny Carroll wrote:
>
>   - Don't use SSD for the Log device.  Write speed tends to be a problem.
It all depends on your usage. You need to experiment, unfortunately.

>   - SSD ok for cache if the sizing is right, but without TRIM, don't
> expect to take full advantage of the SSD.
I do not believe TRIM has any effect on L2ARC.

Why?
- TRIM is a technique to optimize future writes;
- L2ARC is written at controlled, very low rate, I believe something 
like 8MB/sec. There is no SSD currently on the market, with or without 
TRIM that has any trouble sustaining that rate.
- TRIM might introduce delays, it is very 'expensive' command. But that 
will surely wary by drive/manufacturer.
- There is no way TRIM can influence reading from the flash media. 
Reading from L2ARC with low latency and high speed is it's main purpose 
anyway.

> Remaining questions.
> - Is there any advantage to using a spare partition on a SCSI or SATA
> drive as L2Arc?  Assuming it was in the machine already but doing nothing?
Absolutely no advantage. You want L2ARC to be very low latency and 
high-bandwidth for random reading. Especially low-latency. This does not 
apply to rotating disks.

Daniel