Martin Steigerwald <martin@???> wrote:

>> Wear-levelling today is handled by the firmware transparently to the OS.
>> Trimming only affect the filesystem's block-allocator algorithm, not
>> wear-levelling.
>
> How should the drive know that a deleted block is a block is can use again
> without the operating system telling it through trimming?
>
> Wear leveling the firmware of usual consumer SSDs does itself… but it can work
> better when the operating informs it of blocks which got freed by deletion.
> While the firmware can know that an overwritten block is free to use again as
> it always does copy on write… it has no way of knowing whether the filesystem
> freed blocks by deletion of a file without the operating system telling it. So
> trimming does affect the capacity of free space the SSD can use to shuffle
> blocks around.

Exactly.


As an aside, I got asked to look at moving some stuff at work up to Azure. With Azure, you get billed according to the amount of used space in a virtual disk, NOT the size of the disk. I checked this by creating a large file full of zeroes and then deleting it - with the kernel I was running which didn't have TRIM support, Azure then indicated that the billable size had gone up. I didn't get round to upgrading the kernel and retrying it.