[...]
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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