Trim command with SSD 400GB P3600

Hello,

I have an Intel SSD DC P3600 PCIe device. I use ext4 fs and kernel 3.16.0-34 from ubuntu 14.04.2

I would like to ask two questions:

1. What is the preferred way to enable trim on this device -- with the "discard"  mount option or using the scheduled "fstrim" command?

Or  both?

2. If I issue the "fstrim" command, it returns almost immediately. I suppose the SSD device continues discarding after the "fstrim" returns.

How long (upper limit) can it take to trim 320GB or data, assuming that I did modified this data between discards?

$ sudo fstrim -v /mnt/intel_ssd/

/mnt/intel_ssd/: 322074972160 bytes were trimmed

Thank you

P.S.

Device details:

$ sudo isdct show -intelssd

- IntelSSD Index 0 -

Bootloader: 8B1B0129

DevicePath: /dev/nvme0n1

DeviceStatus: Healthy

Firmware: 8DV10130

FirmwareUpdateAvailable: Firmware=8DV10130 Bootloader=8B1B012D

Index: 0

ProductFamily: Intel SSD DC P3600 Series

ModelNumber: INTEL SSDPEDME400G4

SerialNumber: CVMD4381005E400AGN