Re: RAID and SSD configuration question

On Tue, Oct 20, 2015 at 7:30 AM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> On Tue, Oct 20, 2015 at 3:14 AM, Birta Levente <blevi(dot)linux(at)gmail(dot)com> wrote:
>> Hi
>>
>> I have a supermicro SYS-1028R-MCTR, LSI3108 integrated with SuperCap module
>> (BTR-TFM8G-LSICVM02)
>> - 2x300GB 10k spin drive, as raid 1 (OS)
>> - 2x300GB 15k spin drive, as raid 1 (for xlog)
>> - 2x200GB Intel DC S3710 SSD (for DB), as raid 1
>>
>> So how is better for the SSDs: mdraid or controller's raid?
>
> I personally always prefer mdraid if given a choice, especially when
> you have a dedicated boot drive. It's better in DR scenarios and for
> hardware migrations. Personally I find dedicated RAID controllers to
> be baroque. Flash SSDs (at least the good ones) are basically big
> RAID 0s with their own dedicated cache, supercap, and controller
> optimized to the underlying storage peculiarities.
>
>> What's the difference between Write Back and Always Write Back with supercap
>> module?
>
> No clue. With spinning drives simple performance tests would make the
> caching behavior obvious but with SSD that's not always the case. I'm
> guessing(!) 'Always Write Back' allows the controller to buffer writes
> beyond what the devices do.

We're running LSI MegaRAIDs at work with 10 SSD RAID-5 arrays, and we
can get ~5k to 7k tps on a -s 10000 pgbench with the write cache on.

When we turn the write cache off, we get 15k to 20k tps. This is on a
120GB pgbench db that fits in memory, so it's all writes.

Final answer: test it for yourself, you won't know until you do which is faster.