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

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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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).



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