For a good technical overview of how a SSD is implemented inside the SSD itself read http://research.microsoft.com/pubs/63596/USENIX-08-SSD.pdf This whitepaper was written prior to the TRIM functionality being implemented in SSDs, so it does not talk about that. But merely says it would be nice to have a way for the SSD to know when sectors/blocks are being freed by the filesystem. more comments interspersed. On Fri, Jan 16, 2009 at 1:56 AM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > thank you.....but then i don't really understand...please help me out: > > quoting from the article: > > "At the lower levels, groups like the T13 committee (which manages the > ATA standards) have created protocol extensions to allow the host > computer to indicate that certain sectors are no longer in use; T13 > calls its new command "trim." Upon receipt of a trim command, an ATA > device can immediately add the indicated sectors to its free list, > discarding any data stored there. Filesystems, in turn, can cause > these commands to be issued whenever a file is deleted (or truncated). > That will allow the storage device to make full use of the space which > is truly free, making the whole thing work better." > > So my question is: so now the hardware maintain something called a > "freelist"? Yes. Part of the SSDs stable memory is used to track status of each wear leveling unit. ie. To wear level, the SSD has to know how many times the individual erase zones have been erased, which are free, etc. See the MS article for a much better discussion of this. > the interface between software and hardware are the ATA > commands, correct? Yes, as described in the proposed ATA spec you linked at the end of your post. > so anything after the ATA are > firmware-implementation? Yes > so is there not now a duplicated list of > "freelist" of sectors - one at the firmware level, and another at the > filesystem level? Similar, but it is NOT a conceptual problem for SSD to think a sector is in use that the FS thinks is not. It just means the wear-leveling algorithm can't use that sector even though the filesystem would not object. Thus ext2/3 will continue to work with SSDs even though they never discard any sectors. ie. the freelist at the FS level is extremely different than the one in the SSD. BUT the SSD is conservative. Once a logical sector is written, the only way to get the logical sector back into the SSDs free list is for the kernel to "discard" it. > if true, then it also mean a corresponding ATA > command for "allocation" of sectors....correct? No, the FS just writes to a logical sector. If that logical sector does not already have a physical sector assigned, the SSD will allocate one and write to it. Read the MS paper for the gory details of what really happens. It is surprisingly complex. > i think and deduced > these are needed....because of wear leveling requirements as explained > earlier in the article. > > From the ATA spec: > > http://t13.org/Documents/UploadedDocuments/docs2007/e07154r3-Data_Set_Management_Proposal_for_ATA-ACS2.pdf > > it can be understood that the requirements for the TRIM (or deallocate > basically) command is because of frequency statistics needed by the > firmware.....to implement wear leveling i supposed? If I do "dd if=/dev/zero of=/dev/sdc" where sdc is a SSD all of the sectors are allocated and contain real data. How can the wear leveling algorithm work? No free sectors?? Answers are 2: 1) Over build the SSD to always have free sectors, even when it appears full. This is how flash has worked historically. 2) Make the SSD smarter and have the kernel tell the SSD when sectors are no longer needed and available for wear-leveling use. This is the new "trim" feature. Linux is using the term DISCARD. FYI: I have researched this because of my profession. With normal HDDs we have 3 normal things we do: 1) Read all sectors of a drive and recover deleted files. It is not certain, yet but if the SSD manufacturers make the discarded sectors unreadable, this will no longer be feasible via normal sector read requests. 2) Wipe a file to ensure it is not recoverable. (ue. shred file) Due to wear leveling this is not possible. Maybe the shred tool needs to be adjusted to test for SSDs and return a fail code if the file is sitting on one? 3) Wipe a drive Same. Strangely NIST has a document that says it is an acceptable solution for "confidential" data. I can understand that because there is no way I am aware to get to the unallocated sectors of the SSD via the ATA interface. But normally NIST is more paranoid and worries about laboratory attacks. By modifying the SSD I think a good lab would be able to read those unallocated sectors. Greg -- Greg Freemyer Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer First 99 Days Litigation White Paper - http://www.norcrossgroup.com/forms/whitepapers/99%20Days%20whitepaper.pdf The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
