Re: How Do SSDs Wear Out?

On Mon, 2/17/2025 3:46 PM, Carlos E.R. wrote:
> On 2025-02-14 09:11, Paul wrote:
>> On Fri, 2/14/2025 12:47 AM, Boris wrote:
> 
> ...
> 
>> The physical cells, the structure at the atomic level, is
>> damaged by the writes.
>>
>> Each cell has a "voltage" stored on it. Established by putting
>> some electrons on a floating gate. The path for this is
>> quantum mechanically disallowed, and to get the electrons
>> onto the gate requires tunneling. The electrons will sit
>> on the gate for up to ten years (retention time estimate, info
>> on this has not been updated in a long long time so we are left
>> to guess whether it scales in any way with gate size).
> 
> I wonder if we can store the disk for five years, then plug it in and somehow refresh the charges in the cells.
> 
> ...
> 
>> By mapping the sectors, using a mapping table, and "moving the MBR around
>> each time it is written", that is wear leveling. The drive has a pool of
>> unwritten blocks. On a write request, an unused block is written.
>> Perhaps the block is at address 27, and it contained MBR sector 0.
>> The map file the drive keeps then, it has to remember that aspect.
>> On a read, we request sector 0, the map goes "oh, that is block 27",
>> and the drive does the read at that address, and there is our MBR.
>> Now, if I abuse the MBR by writing it a lot, a hole isn't burned in it.
>> The sector has been "virtualized", and only the mapping table knows
>> where my sector is stored :-)
> 
> Where is the map stored? I always wondered about this.
> 
> ...
> 
> 
> Thanks a lot for the summary :-)

The issue of "recharging cells" has already come up, with respect
to TLC. For at least Samsung, they may have a provision for doing that.

SSDs don't have a real time clock, so they cannot tell five years has
passed. All they have is power-on-hours, which is a useful metric for
an SSD that is alive and working every single day.

The drive can tell when a sector is getting "spongy" due to the
error count. A TLC sector, might have a bit in error, for every sector.
Correcting all the sectors is nothing new for the drive. And since
the syndrome is 50 bytes for a 512 byte sector, that's a *huge* syndrome
allowing a lot of bits in error to be corrected. The drive can allow
the TLC cell to have more and more errors in it. Then, once a portion
of the error capability is used up, the drive could re-write the sector.

That's one way they could do it.

But since no company has enthusiast promoters like in the OCZ days,
we cannot get information from company reps about how things work.

*******

The map can be stored in an "SLC-like" critical data storage
area of the flash. But that's not the part I am particularly
interested in. I'm more curious about how a map file can be
maintained, without burning a hole in the SSD while doing so.
It is the handling policy of the map file, whether it is journaled
or protected in some way, that I am curious about.

But don't expect to find an honest explainer page on the web.

All we know, is the power can go off, and the SSD drive seems to survive.

   Paul