Re: put 2 hard drives in mdadm raid 1 and detect bitrot like btrfs does, what's that called?

[...]

Hey Andy,

I would rather see performance figures for these setups:
A) btrfs with 2 (or more) hard drives and one SSD in writeback bcache configuration (unsafe against failure of the ssd):
+-----------------------------+
|      btrfs raid 1 /mnt      |
+--------------+--------------+
| /dev/Bcache0 | /dev/Bcache1 |
+--------------+--------------+
|   bcache writeback Cache    |
|           /dev/sdk1         |
+--------------+--------------+
| Data         | Data         |
| /dev/sdv1    | /dev/sdw1    |
+--------------+--------------+

Doing that, you loose the protection of raid1 redundancy: now there is a single point of failure /dev/sdk1. Writeback is even more dangerous...

B) btrfs with 2 (or more) hard drives and two SSD's in dm-raid 1 writeback bcache configuration (unsafe against corruption of any of the ssd's):
+-----------------------------+
|      btrfs raid 1 /mnt      |
+--------------+--------------+
| /dev/Bcache0 | /dev/Bcache1 |
+--------------+--------------+
|   bcache writeback Cache    |
|           /dev/dm0          |
+--------------+--------------+
| 2x SSD in mdadm raid 1      |
| /dev/sdk1       /dev/sdl1   |
+--------------+--------------+
| Data         | Data         |
| /dev/sdv1    | /dev/sdw1    |
+--------------+--------------+

C) Full stack: btrfs with 2 (or more) hard drives and two identical SSD's in dm-raid 1 with dm-integrity writeback bcache configuration (safe against any failed drive):
+-----------------------------+
|      btrfs raid 1 /mnt      |
+--------------+--------------+
| /dev/Bcache0 | /dev/Bcache1 |
+--------------+--------------+
|   bcache writeback Cache    |
|           /dev/dm0          |
+--------------+--------------+
| 2 x dm-integrity devices    |
| in mdadm raid 1             |
+--------------+--------------+
| SSD hosting  | SSD hosting  |
| dm-integrity | dm-integrity |
| /dev/sdk1    | /dev/sdl1    |
+--------------+--------------+
| Data         | Data         |
| /dev/sdv1    | /dev/sdw1    |
+--------------+--------------+

D) Full stack: btrfs with 2 (or more) hard drives and two SSD's (one slow, and one very fast) in dm-raid 1 with dm-integrity writeback bcache configuration (safe against any failed drive):
+-----------------------------+
|      btrfs raid 1 /mnt      |
+--------------+--------------+
| /dev/Bcache0 | /dev/Bcache1 |
+--------------+--------------+
|   bcache writeback Cache    |
|           /dev/dm0          |
+--------------+--------------+
| 2 x dm-integrity devices    |
| in mdadm raid 1             |
+--------------+--------------+
| SSD hosting  | SSD hosting  |
| dm-integrity | dm-integrity |
| /dev/sdk1    | /dev/sdl1    |
+--------------+--------------+
| Data         | Data         |
| /dev/sdv1    | /dev/sdw1    |
+--------------+--------------+

In all these setups, the performance of the hard drives is irrelevant, because the speed of the setups comes from the bcache SSD.

Cheers,
Cedric

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