Re: Experiences on BTRFS Dual SSD RAID 1 with outage of one SSD

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

On 2018-08-17 08:50, Roman Mamedov wrote:
> On Fri, 17 Aug 2018 14:28:25 +0200
> Martin Steigerwald <martin@lichtvoll.de> wrote:
> 
>>> First off, keep in mind that the SSD firmware doing compression only
>>> really helps with wear-leveling.  Doing it in the filesystem will help
>>> not only with that, but will also give you more space to work with.
>>
>> While also reducing the ability of the SSD to wear-level. The more data
>> I fit on the SSD, the less it can wear-level. And the better I compress
>> that data, the less it can wear-level.
> 
> Do not consider SSD "compression" as a factor in any of your calculations or
> planning. Modern controllers do not do it anymore, the last ones that did are
> SandForce, and that's 2010 era stuff. You can check for yourself by comparing
> write speeds of compressible vs incompressible data, it should be the same. At
> most, the modern ones know to recognize a stream of binary zeroes and have a
> special case for that.
All that testing write speeds forz compressible versus incompressible 
data tells you is if the SSD is doing real-time compression of data, not 
if they are doing any compression at all..  Also, this test only works 
if you turn the write-cache on the device off.

Besides, you can't prove 100% for certain that any manufacturer who does 
not sell their controller chips isn't doing this, which means there are 
a few manufacturers that may still be doing it.