Disk trimming is the procedure by which the operating system informs the underlying storage device of which storage blocks are no longer in use.
It does this by issuing an `ATA_TRIM` command for the block. This is also known as a `discard`.
In this way, the storage device can perform garbage collection of the unused blocks and internally prepare them for reuse. SSDs in general benefit from this, while HDDs do not.
In a Linux system running on bare metal, this is relatively straight-forward.
When instructed by the operating system, discards are issued by the file-system driver directly to the storage driver and then to the SSD.
In Qubes, this gets more complex due to virtualization, LUKS, and LVM (and thin pools on R4.0 and up).
If discards are not supported at any one of those layers, it will not make it to the underlying physical device.
There are some security implications to permitting TRIM (read for example [this article](https://asalor.blogspot.com/2011/08/trim-dm-crypt-problems.html)), but in most cases not exploitable.
Conversely, TRIM can improve security against local forensics when using SSDs, because with TRIM enabled deleting data (usually) results in the actual data being erased quickly, rather than remaining in unallocated space indefinitely.
However deletion is not guaranteed, and can fail to happen without warning for a variety of reasons.
**Note** Although discards can be issued on every delete inside `dom0` by adding the `discard` mount option to `/etc/fstab`, this option can hurt performance so the above procedure is recommended instead.
However, inside App and Template qubes, the `discard` mount option is on by default to notify the LVM thin pool driver (R4.0) or sparse file driver (R3.2) that the space is no longer needed and can be zeroed and re-used.
In R4.x, LVM Logical volumes are frequently deleted (every time a disposable VM is shut down, for example) so you may want to set `issue_discards = 1` if using an SSD, but see the article linked in the first section of this page.
5. To verify if discards are enabled you may use `dmsetup table` (confirm the line for your device mentions "discards") or just run `fstrim -av` (you should see a `/` followed by the number of bytes trimmed).
To enable it add the `discard` flag to the options for the swap entry in `/etc/fstab`.
This may or may not actually improve performance.
If you only want the security against local forensics benefit of TRIM, you can use the `discard=once` option instead to only perform the TRIM operation once during at boot.
To verify that TRIM is enabled, check `dmesg` for what flags were enabled when the swap space was activated.
You should see something like the following:
Adding 32391164k swap on /dev/mapper/qubes_dom0-swap. Priority:-2 extents:1 across:32391164k SSDscFS
The `s` indicates that the entire swap device will be trimmed at boot, and `c` indicates that individual pages are trimmed after they are no longer being used.
