Several topics in this board point out the recommendation about leaving 10 % - 25 % of free space when using SSD-drives.
The central question now is:
Does this 25 % recommendation refer to a completely unformatted blank rest space on the disk? (= unformatted raw partition)
or
Is it okay to format the whole disk with a filesystem in complete disk size, but later just pay attention to avoid filling up the filesystem with data completely (= leaving 25 % empty space inside the formated partition)?
You don't operate directly on the flash layer of the drive, you work on a layer above the wear leveller so the two cases you mention are functionally the same.
If it is a brand new drive and you know for certain that it has never been used then the two cases will be identical.
If you have used the drive fully partitioned then you cannot guarantee where the operating system will have allocated on the drive, in this case continuing to use a full partition would be better as the operating system can issue TRIM commands to clear the empty space.
TRIM commands specifically tell the wear leveller what blocks have been made empty and can help the drive make intelligent choices about where and what to move so having the operating system make it absolutely clear what blocks can be erased without consequence could make things better in the long run, especially if you allocated and used the space and then just deleted the partition.
I've posted more on the Flash Transition Layer over here.
This recommendation relates to the entire number of sectors on the disk. The reason behind it is that the disk's firmware needs some free space for garbage collection. The firmware does not really care or know about formatted or unformatted space or about partitioning.
The number of 25% is without scientific basis and may be exaggerated.
The answers to your question are :
Does this 25 % recommendation refer to a completely unformatted blank rest space on the disk? (= unformatted raw partition)
Yes - to the entire SSD. An unformatted partition is part of this space.
Is it okay to format the whole disk with a filesystem in complete disk size, but later just pay attention to avoid filling up the filesystem with data completely (= leaving 25 % empty space inside the formated partition)?
Yes, this is the same thing. You should format an SSD using quick format, as there is no point in writing over every sector.
First of all, the 25% is not the same for every drive, but the principle remains the same. Some drives can use as little as 5% before getting in serious performance issues.
This is for a drive where you have a full partition spanning everything and then keep 25% of that partition free of data.
Although I'm pretty sure you can also use a smaller partition and it will work the same.
The reason is that the disk has small clusters of data. When data is written to these, its not always the case that these clusters are fully written to. So the clusters may be only filled partially (will happen with many small files written, then deleted.)
The drive will periodically execute a TRIM command which will find clusters that have been cleared of data. When that happens, that cluster is entirely marked as free, and it is this principle that makes SSD's fast. When too little room on an SSD is free, when new data is written to the SSD, there are no empty clusters anymore, and data is forced to be written to clusters that already have data in them, but is not fully used. This means the disk becomes very fragmented, and that severely impacts the performance of the drive.
The partition scheme does not impact where data is physically stored on the SSD, so that's why I'm pretty sure either will work fine.
While functionally equivalent, I prefer to leave RAW space at the end so that if a user fills the formatted partion(s), they have a buffer and basically a built-in warning that the drive is filling beyond a safe level.
Especially helpful with applications that log continuously, and if something were to go awry and un-monitored the application could fill the partition but not break the hardware.
