Re: [gentoo-user] Finally got a SSD drive to put my OS on

[Frank Steinmetzger <Warp_7@gmx.de>]

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Am Sun, Apr 16, 2023 at 08:08:59AM -0700 schrieb Mark Knecht:

> If you have an SSD or nvme drive installed then fstrim should be
> installed and run on a regular basis. However it's not 'required'.
> 
> Your system will still work, but after all blocks on the drive have
> been used for file storage and later deleted, if they are not
> written back to zeros then the next time you go to use that
> block the write will be slower as the write must first write
> zeros and then your data.
> 
> fstrim does the write to zeros so that during normal operation
> you don't wait.

That is not quite correct. Trimming is about the oppisite of what you say, 
namely to *not* rewrite areas. Flash memory can only be written to in 
relatively large blocks. So if your file system wants to write 4 KiB, the 
drive needs to read all the many kB around it (several hundreds at least, 
perhaps eben MiBs, I’m not certain), change the small part in question and 
write the whole block back. This is called write amplification. This also 
occurs on hard drives, for example when you run a database which uses 4 kiB 
datafile chunks, but on a file system with larger sectors. Then the file 
system is the cause for write amplification.

If the SSD knew beforehand that the area is unused, it does not need to read 
it all in and then write it back. The SSD controller has no knowledge of 
file systems. And this is where trim comes in: it does know file systems, 
detects the unused areas and translates that info for the drive controller. 
Also, only trimmed areas (i.e. areas the controller knows are unused) can be 
used for wear leveling.

I even think that If you read from a trimmed area, the controller does not 
actually read the flash device, but simply returns zeroes. This is basically 
what a quick erase does; it trims the entire drive, which takes only a few 
seconds, and then all the data has become inaccessible (unless you address 
the memory chips directly). It is similar to deleting a file: you erase its 
entry in the directory, but not the actual payload bytes.

AFAIK, SMR HDDs also support trim these days, so they don’t need to do their 
SMR reshuffling. I have a WD Passport Ultra external 2.5″ HDD with 5 TB, and 
it supports trim. However, a WD Elements 2.5″ 4 TB does not. Perhaps because 
it is a cheaper series. Every laptop HDD of 2 (or even 1) TB is SMR.

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