Linux – Possible to get SSD TRIM (discard) working on ext4 + LVM + software RAID in Linux

You don't want to use dmraid unless you have specific motherboard or controller and understand the ramifications of the so-called fakeRAID.

It appears you are thinking about lvm mirrors (which, of course can use device mapper--and thus the potential for confusion with dmraid--something else entirely).

The problem with using lvm mirrors is you are mirroring entire physical extents--typically sized in megabytes--nowhere near the block-level mirroring done with md.

That said, I've done it. But only on short-term back-end migration scenarios. And recovery from a failure is a bit more tricky as it's not as magically automatic as proper md raid1.

As far as I know, TRIM won't be passed to SSDs in hardware RAID. It's not so much that the lack of TRIM will affect the lifetime of an SSD (although this may be a factor), but that the drive's performance will suffer relatively quickly - which somewhat contradicts the purpose of the SSD.

If you really need RAID 1, then I'd favour SandForce-based SSDs, which tend to cope better without TRIM.

Edit: It's difficult to quantify how much the MTBF would reduce without TRIM, without being an SSD manfacturer. But we do know that the lack of TRIM can increase write amplification, thereby shortening the life of the drive due to extra program-erase cycles. SandForce drives have really low write amplification rates anyway - typically less than 1.0, due to compression - which explains why they cope better than other SSDs without TRIM.

It's also important to clarify that there are two types of SSD failure:

1. Wear of the flash memory due to excessive program-erase cycles over time
2. Failure of the SSD controller hardware or software

It is most likely the second of these which is described here, and which accounts for the relatively high failure rates here (albeit, presumably for consumer drives). In these cases, because failure occurs relatively early, it's unlikely that TRIM would make any difference to the reliability of the drive.