Date: Wed, 09 Jul 2014 12:27:23 +0300 From: Daniel Kalchev <daniel@digsys.bg> To: freebsd-fs@freebsd.org Subject: Re: Using 2 SSD's to create a SLOG Message-ID: <53BD0AFB.3000909@digsys.bg> In-Reply-To: <CAP1HOmRe5hEts6qA=0ZiNbMt%2B%2BDoyJO1f0fs=Wnx4ceC%2BygWeg@mail.gmail.com> References: <CAP1HOmRe5hEts6qA=0ZiNbMt%2B%2BDoyJO1f0fs=Wnx4ceC%2BygWeg@mail.gmail.com>
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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
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