Block device discard

Firecracker can expose virtio-blk discard support to Linux guests. When enabled, the guest can issue discard/TRIM requests, for example through fstrim, and Firecracker forwards those requests to the backing storage.

Discard is configured per virtio-block device through the discard field in the PUT /drives/{drive_id} request. It is disabled by default.

Supported configuration

Discard is currently supported only for writable virtio-block devices using the Sync IO engine. It is not supported for:

• read-only drives;
• Async IO engine drives;
• vhost-user block devices.

For regular backing files, Firecracker uses hole punching. For block-device backends, Firecracker uses BLKDISCARD.

Discard alignment and drive updates

The discard alignment is measured in 512-byte sectors. Firecracker sets it to the advertised logical block size divided by the 512-byte sector size.

For a disk update, the new backend discard alignment must divide the advertised alignment. Firecracker does not check this requirement, so the user must provide a compatible backend. Otherwise, the backend can reject a valid guest request, and Firecracker returns VIRTIO_BLK_S_IOERR. For example, replacing a regular file with alignment 1 by a block file with 4096-byte logical sectors having alignment 8 is invalid because 1 % 8 != 0.

Example configuration

curl --unix-socket ${socket} -i \
     -X PUT "http://localhost/drives/rootfs" \
     -H "accept: application/json" \
     -H "Content-Type: application/json" \
     -d "{
             \"drive_id\": \"rootfs\",
             \"path_on_host\": \"${drive_path}\",
             \"is_root_device\": true,
             \"is_read_only\": false,
             \"discard\": true,
             \"io_engine\": \"Sync\"
         }"

After the guest boots, Linux guests can usually issue discard requests with:

fstrim -av