Mirrored SSDs for ZIL/SLOG - safety, flushing, capacitors

[Gary Palmer]

On Fri, Oct 03, 2014 at 05:59:48AM -0600, Warren Block wrote:
> On Thu, 2 Oct 2014, javocado wrote:
> 
> > I'm setting up a SLOG to improve performance on my ZFS-based fileserver. I
> > plan to use 2 mirrored SSD's to create the log device. Before I continue,
> > I have some specific questions how to approach which hardware to use.
> >
> > Primarily, I'm concerned with what damage/corruption may occur when there's
> > a power loss.
> 
> Do you have a good UPS?
> 
> > 2. should I only use SSDs with capacitors? does anyone recommend any? I'm
> > seeing good things about the Intel s3500
> 
> I think the Crucial M550 SSDs have additional backup capacitors also. 
> However, I'm still skeptical about the value of a small SSD power backup 
> versus a true UPS for the entire system.

The point of the supercap is all SSDs (even the ones without proper power
protection) have a small amount of DRAM which they use for write optimisation.
The cap is just there to let the SSD dump the DRAM to flash if the power dies.

Even with a good UPS you should still get SSDs with power protection in case
of PSU failures or people knocking out power cords or something.  

Regards,

Gary