[riobard]

One thing I'm still puzzled about SSD over-provisioning, which is also mentioned by the tutorial (https://codecapsule.com/2014/02/12/coding-for-ssds-part-4-ad...) recommended by the article:

> A drive can be over-provisioned simply by formatting it to a logical partition capacity smaller than the maximum physical capacity. The remaining space, invisible to the user, will still be visible and used by the SSD controller.

Does the controller read the partition table to decide that the space beyond logic partition is safe to use as scrap?

[rdc12]

The SSD maintains a translation table for all the virtual addresses exposed by the drive, that maps to the underlying flash physical addresses. Any physical address not in that table, is unallocated and the drive can use freely.

[riobard]

So over-provisioning has to be done before any writes to the drive? What if I want to over-provision a used drive? Discard all blocks first?

[wtallis]

With most SSDs, there's no special explicit step necessary to overprovision a device. Just trim/unmap/discard a range of logical block addresses, and then never touch them again. The drive won't have any live data to preserve for those LBAs after they've been wiped by the trip operation, and the total amount of live data it is tracking will stay well below the advertised capacity of the drive.

The easiest way to achieve this is to create a partition with no filesystem, and use blkdiscard or similar to trim the LBAs corresponding to that partition.

[ars]

Any sector with nothing written on it can be used as scrap.

So if you partition the entire thing, but just never write to the full disk (you never use all the space), that also works as overprovisioning.

Partitioning just forces that to happen.

[riobard]

If I partition the entire drive, eventually all blocks will be used, depending on how the filesystem allocates, right? So to guarantee some free space it's better to over-provision by under-partitioning. Now how do I make sure that on a used drive?

[rdc12]

You could use some sort of disk quota system, to make the filesystem artificially smaller than it actually is (trim after applying this change). Or simply insure that you don't exceed 80% - 90% used space.

It it is also worth noting that many SSD's are over-provisioned by the manufacture anyway, in those drives manual over-provisioning might achieve very little anyway.

[ars]

That's what the trim command does.

It runs periodically and lets the SSD know about unused areas.

So as long as you don't fill up the drive and let trim do its thing the unused areas effectively do the same thing as over provisioning.