Am 06.05.2013 13:00, schrieb Hinnerk van Bruinehsen: > On Mon, May 06, 2013 at 07:50:52AM +0100, Stroller wrote: >> On 5 May 2013, at 17:16, Hinnerk van Bruinehsen wrote: >>> ... The data on a SSD is not >>> necessarily stored linar so it's not said that a new partition is using >>> the same memory cells as the old one. >>> … >>> For a HDD I'd advise to create a copy >>> using dd but from my understanding of SSD technology it's not >>> guaranteed to copy the right (now unused marked) blocks. >> Is anyone able to elaborate on this, please? >> >> I think I've had a eureka! moment of understanding whilst preparing to >> compose this reply, but I've always been sceptical of these kinds of >> statements in the past. >> >> Surely flash memory devices must present themselves to the o/s as block >> devices, because that's how all storage devices work, right? >> >> If I'm now understanding correctly, SSDs present themselves to the o/s as >> block devices more or less as convenient or necessary. They can be treated >> as such as long as all the data required is listed in the file allocation >> table. I'm left wondering how the SSD knows that a file has been deleted, >> and whether this works for all conceivable file-systems. > The problem is that you can't delete on a flash cell. The process is > simplified: read cell - delete to be deleted stuff in memory - write > memory contents back. > > Since flash cells can only be written to a fixed amount of times > (afterwards they become unreliable) there is a concept called wear > leveling. This means essentially that your 128 GB flash drive in reality > hasn't just 128 GB of storage but e.g. 256GB. To spread out the writes > it reads one cell, does the memory operation and write the contents back > to another cell while marking the old cell as unused.
emm - no. Wear leveling does not need any spare blocks. A lot of drives do have spare blocks, but those are never the same size of the original size (at least not on drives you can buy for a sensible amount of money). More like 120+8 or 160+16 or 256+16. The spare blocks are used like on a hdd: some block goes bad, another one is mapped in. Since the sdd firmware does not know if something was deleted or not* - it does know shit about filesystems**, you can of course dd an image, if you want to. Just like on a hdd. *there are drives that do garbage collection without TRIM for fat and or ntfs.. so they seem to know a bit about filesystems. ** and this is why TRIM exists in the first place. To tell the drive: yes, this data is gone. You don't need to care about it anymore.
