A lot of SSD-related instructions online currently say you should add allow-discards and root_trim=yes to your GRUB_CMDLINE_LINUX. I have yet to find one that says why you should do that, i.e. what exactly (if anything!) do those parameters do. Where is the documentation on this and what does it say about those two parameters' purpose?
According to Cryptsetup 1.4.0 Release Notes,
Since kernel 3.1, dm-crypt devices optionally (not by default) support block discards (TRIM) commands. If you want to enable this operation, you have to enable it manually on every activation using --allow-discards
cryptsetup luksOpen --allow-discards /dev/sdb test_disk
but is it the same when passed to the kernel (via GRUB_CMDLINE_LINUX)?
Edit: Kernel.org's list of kernel parameters doesn't (currently, Jan 2013, at least) have either of these options.