Using 2 SSD's to create a SLOG

[javocado]

Thank you for your feedback. We understand that the devices are mirrored
and tolerant.
To further my understanding, confirm that I should be able to yank one of
the SLOG SSD drives and zfs won't care, and I can subsequently replace it
(issuing what command?) and zfs will then re-mirror it? And if both drives
are removed then only the sync data would be lost? But if the drives are
restored, or in the case of a power loss when the server comes back up, the
sync data on the SSD's will be processed once zfs starts?



On Wed, Jul 9, 2014 at 2:27 AM, Daniel Kalchev <daniel at digsys.bg> wrote:

>
> 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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