Re: RAID and SSD configuration question

Hi,

On 10/20/2015 03:30 PM, Merlin Moncure 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.

I don't know - I've always treated mdraid with a bit of suspicion as it
does not have any "global" write cache, which might be allowing failure
modes akin to the RAID5 write hole (similar issues exist for non-parity
RAID levels like RAID-1 or RAID-10).

I don't think the write cache on the devices prevents this, as it does
not prevent problems with interruption between writes the two drives.

>
>> What's the difference between Write Back and Always Write Back
>> withsupercap 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.

AFAIK there's no difference. It's an option that disables write cache in
case the battery on BBU dies for some reason (so the write cache would
become volatile). With capacitors this is not really applicable.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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