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Interesting. But that would only be true for journaling file systems right? Or are there consumer hard disk drives that transparently journal writes a la ssd's?

by mustpax 16y ago

pmorici 16y ago
Newer generations of SSD's have a command called "TRIM". It is meant as a way to speed up SSD drives by alerting the drive to blocks that are no longer in use by the file system and can be pre erased so to speak which makes future writes to that location faster. Windows 7 and newer versions of the Linux kernel support it.
- chrisbolt 16y ago
  That doesn't necessarily mean it will be erased, it just gets put back in the list of unallocated blocks.
  - Someone 16y ago
    No. Flash memory is sort-of write-once in the sense that a page starts with all bits in one state. From there, you can flip individual bits to the other state, but to flip them back, you have to flip back all of them. So, to update a single bit, you may have to read a block, erase it, update the in-memory block, then write the in-memory block. The trim command will do the 'erase block' part of this procedure before the block is actually needed for data. That speeds up a future write to that block.
tzs 16y ago
It's not just journaling that is a problem. Some filesystems and/or operating systems will move files or parts of files to improve performance. For example OS X does some automatic defragmentation of files. Some systems move files that are frequently accessed together to be near each other on the disk. When the system does such an automatic move of a file or part of a file, does it clear out the source block on the disk, or just mark it free? If the latter, then a secure erase that just overwrites the current blocks of the file will leave those other blocks untouched in the free space. If you have a file that you must truly be able to get rid of without destroying your whole hard disk, that file should be kept encrypted on the disk, and the plaintext version should never be stored on disk. Getting rid of the file is then a simple matter of forgetting the encryption key.

Interesting. But that would only be true for journaling file systems right? Or are there consumer hard disk drives that transparently journal writes a la ssd's?

by mustpax 16y ago

Interesting. But that would only be true for journaling file systems right? Or are there consumer hard disk drives that transparently journal writes a la ssd's?

pmorici 16y ago

Newer generations of SSD's have a command called "TRIM". It is meant as a way to speed up SSD drives by alerting the drive to blocks that are no longer in use by the file system and can be pre erased so to speak which makes future writes to that location faster. Windows 7 and newer versions of the Linux kernel support it.

chrisbolt 16y ago
That doesn't necessarily mean it will be erased, it just gets put back in the list of unallocated blocks.
- Someone 16y ago
  No. Flash memory is sort-of write-once in the sense that a page starts with all bits in one state. From there, you can flip individual bits to the other state, but to flip them back, you have to flip back all of them. So, to update a single bit, you may have to read a block, erase it, update the in-memory block, then write the in-memory block. The trim command will do the 'erase block' part of this procedure before the block is actually needed for data. That speeds up a future write to that block.

tzs 16y ago

It's not just journaling that is a problem. Some filesystems and/or operating systems will move files or parts of files to improve performance. For example OS X does some automatic defragmentation of files. Some systems move files that are frequently accessed together to be near each other on the disk. When the system does such an automatic move of a file or part of a file, does it clear out the source block on the disk, or just mark it free? If the latter, then a secure erase that just overwrites the current blocks of the file will leave those other blocks untouched in the free space. If you have a file that you must truly be able to get rid of without destroying your whole hard disk, that file should be kept encrypted on the disk, and the plaintext version should never be stored on disk. Getting rid of the file is then a simple matter of forgetting the encryption key.