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Re: how to enable trim for an external encrypted SSD?




On 07.11.2017 01:43, Michael Stone wrote:
On Tue, Nov 07, 2017 at 01:00:54AM +0500, Alexander V. Makartsev wrote:
Now I wonder, why would anyone want to use TRIM feature on a portable SSD,
especially on an encrypted one?
Isn't this feature only increases drive wear, offering next to nothing as a
benefit in return?
I don't use TRIM feature on my systems because of that, and hope for a firmware
of modern SSD will manage everything automatically. Am I wrong?

TRIM should reduce, not increase, drive wear. (Since the drive knows that a particular block is unusued, it can write directly instead of having to read and rewrite an entire erase group.)
As I know SSDs are different in a way HDDs write their data. HDDs write directly to the free block and are done. SSDs on the other hand will have to erase free block first to get it ready for writing and then write to it. This is because how NAND work. It's even worse, if a destination block has some data on it already and there is some data needs to be written onto it, then SSD will copy data already written, then it will fully erase destination block, then it will concatenate copied data with data to be written and finally will write merged data to the block.
TRIM will erase blocks marked as deleted beforehand even if they are not needed for writing right now. This overhead is the reason that causes increased drive wear and decrease it's lifespan.

I do agree, that using fstune manually once in say half a year on a non busy SSD could improve writing performance for some time, but using discard or equivalent permanently in fstab is hardly worth it.

-- 
With kindest regards, Alexander.

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