dm-writeboost + MD + LUKS?

[kamild1996]

Looking for a way to improve write speeds on my home NAS. I have two HDDs setup in software mirror RAID (mdraid) with LUKS encryption on top of that. Is dm-writeboost capable of working with such configured drives? And if yes - which device do I point as the backing device, the encrypted one (/dev/dm-0), or the decrypted one (/dev/mapper/md127)?

[akiradeveloper]

@kamild1996

What is your configuration? Is this correct?

• /dev/dm-0 (crypted)
  • /dev/mapper/md127 (software RAID)
    • HDD1
    • HDD2

[kamild1996]

Yes. Both drives are merged in software RAID into dm-0, and md127 is dm-0 but decrypted by LUKS and ready to use.

[akiradeveloper]

I see. So this configuration should be correct.

• /dev/mapper/md127 (LUKS)
  • /dev/dm-0 (encrypted, software RAID)
    • HDD1
    • HDD2

[kamild1996]

Yep, that's precisely it.

[akiradeveloper]

If you don't want decrypted data to be found on SSD cache device, you have to stack LUKS on top of everything. Otherwise, my recommendation is stacking writeboost on the very top. Here are the reasons:

1. LUKS on top of writeboost is ever reported a bug (but it is not reproduced and thought to be resolved). 2.2.4: corruption with cryptdisk on top of writeboost #111
2. In practice, you should put most well-tested module at the bottom of the stack. Unfortunately, writeboost can not be said to be well-tested relative to LUKS and RAID1 and the technical difficulty is much higher with writeboost.
3. If you put LUKS on top of writeboost, the incoming write to writeboost is limited by LUKS, leading to inefficient use of SSD write caching.

[kamild1996]

Thank you a lot for a detailed answer @akiradeveloper - I think I'm fine with cache contents being unencrypted, so I'll go with dm-writeboost on top, pointed at /dev/mapper/md127.