Re: mkfs.btrfs cannot find rotational file for SSD detection for a pmem device

[Boaz Harrosh <boaz@plexistor.com>]

On 09/09/2015 03:40 PM, Austin S Hemmelgarn wrote:
> On 2015-09-09 08:12, Boaz Harrosh wrote:
>> On 09/09/2015 02:28 PM, Austin S Hemmelgarn wrote:
>>> On 2015-09-08 16:00, Elliott, Robert (Persistent Memory) wrote:
>> <>
>>
>>> this may actually make things slower (the particular effect of SSD mode
>>> is that it tries to spread allocations out as much as possible, as this
>>> helps with wear-leveling on many SSD's).
>>>
>>
>> For DRAM based NvDIMM it matters not at all. For Flash based or the new
>> 3d Xpoint it is a plus, so no harm in leaving it in
>>
> Looking at it from another perspective however, a lot of modern RAM 
> modules will stripe the bits across multiple chips to improve 
> performance.  In such a situation, BTRFS making the effort to spread out 
> the allocation as much as possible may have an impact because that 
> allocation path is slower than the regular one (not by much, but even a 
> few microseconds can make a difference when it is getting called a lot).
> 
> 

It is pointless to argue about this, but the allocations are 4k aligned
any which way, which means you are right at the beginning of the striping,
the RAM striping is a cacheline (64 bytes) granularity.

I think you will never find a single micro benchmark that will ever produce
a difference.

Cheers
Boaz