(1) Automatic and selective wiping of unused and previously used disk
blocks is a good security measure, particularly when there is an
encryption layer beneath the file system.
(2) USB attached devices _never_ support TRIM and they are the most
likely to fall into strangers hands.
(3) I vaguely recall that some flash chips will take bulk writhes of
full sectors of 0x00 or 0xFF (I don't remember which) were second-best
to TRIM for letting the flash controllers defragment their internals.
So it would be dog-slow, but it would be neat if BTRFS had a mount
option to convert any TRIM command from above into the write of a zero,
0xFF, or trash block to the device below if that device doesn't support
TRIM. Real TRIM support would override the block write.
Obviously doing an fstrim would involve a lot of slow device writes but
only for people likely to do that sort of thing.
For testing purposes the destruction of unused pages in this manner
might catch file system failures or coding errors.
(The other layer where this might be most appropriate is in cryptsetup
et al, where it could lie about TRIM support, but that sort of stealth
lag might be bad for filesystem-level operations. Doing it there would
also loose the simpler USB use cases.)
...Just a thought...
--Rob White.
