Two SSDs in a 'single' cache pool - if one dies, is the data on the other accessible?

[Nirin]

I want two SSDs to be merged into a single pool. I don't care about parity, and they are backed up separately. 

 

The only ways to do this seems to be either Raid0 (faster, but if one dies the data on both is lost), or having them both balanced as 'single' which (from what I can tell) is a JBOD setup. 

 

But there seems to be some ambiguity... I've seen some say the data fills up the drives like two separate volumes, and so if one dies the data on the other is still readable, while others say the spanning happens in such a way that if one drive dies, the data on the other is gone, making it no different from a raid0 (except maybe by using special recovery services). 

 

As in either case it's probably easier to recover from the backup anyway, it's mostly an academic question (I got curious and went down a rabbit hole). But if there's zero benefit to using the 'single' option then it seems Raid0 might as well be used (even though I generally steer clear of 0).

[JorgeB]

2 minutes ago, Nirin said:

while others say the spanning happens in such a way that if one drive dies, the data on the other is gone, making it no different from a raid0 (except maybe by using special recovery services). 

This is correct, with btrfs single profile the data is split in 1GiB chunks, and writes alternates between the devices, assuming they start with the same available space.

[Nirin]

26 minutes ago, JorgeB said:

This is correct, with btrfs single profile the data is split in 1GiB chunks, and writes alternates between the devices, assuming they start with the same available space.

 

Ahh I see, that makes sense. Interesting. So there's not really any point in using it over a raid0.

[JorgeB]

1 hour ago, Nirin said:

So there's not really any point in using it over a raid0.

 

Only if you have different capacity devices, in single mode you can fully use both, other than that it will just be slower than raid0 and have the same risk of losing the data.