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Date: Wed, 09 Jul 2014 12:27:23 +0300
From: Daniel Kalchev <daniel@digsys.bg>
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Subject: Re: Using 2 SSD's to create a SLOG
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On 08.07.14 04:06, javocado wrote:
> I am hoping to dumbly plug two SSDs onto motherboard SATA ports and just
> hand them over, raw, to ZFS.
>

Others already commented how you should setup mirrored SLOG. In addition 
to that, because of the nature of SSDs and SLOG, I would recommend the 
following:

The SLOG size does not need to be large, it should only cover several 
seconds of your synchronous write throughput -- usually few GB are 
plenty. Today's SSDs are much large than needed for SLOG. But, today's 
SSDs also suffer severe performance degradation, especially for writing 
when you fill them up with data and they need to do garbage collection. 
Also, most SSDs have "good performance" only when using an 8GB span, not 
the whole drive. All of this only makes sense if the drive has TRIM. 
FreeBSD already supports TRIM for ZFS SLOG. Therefore ensure you do TRIM 
of the entire drive, then partition it with GPT to only use (say) 8GB 
for the SLOG. Leave the rest unallocated -- you will never write there 
but the drive's controller will use those blocks as spares for TRIM and 
this will both improve performance and make the drive last much longer. 
Then add both slices as a mirrored log device to your ZFS pool.

Daniel