On Thu, 4 May 2017 09:55:51 +0200
Andreas Klauer [off-list ref] wrote:

For example? I was under the impression that pretty much all of them do. 
Even the ones that don't advertize it returned zero after trim for me. 
[Sometimes you get original data but that's Linux; gone after drop_caches]
(Of course, I don't have access to that many different models of SSD...)

But what do they actually return then? Original data?

Consult Wikipedia at least, if not the original ATA standards documents:
https://en.wikipedia.org/wiki/Trim_(computing)#ATA
Can return "undefined", "something that is the same every time" or "zeroes".
Apparently the OS can query which is to be expected with a particular device,
but firstly, you can't necessarily trust that 100%, and secondly, that's
operating on a level lower (ATA) than where md is.

-- 
With respect,
Roman