Thanks Hugo, Since:
-- i keep daily backups -- all 4 devices are of the same size I think I can test it (as soon as I have some time to spend in the transition to BTRFS) and verify your assumptions (...and get my wish) > If you have an even number of devices and all the devices are the > same size, then: > > * the block group allocator will use all the devices each time > * the amount of space on each device will always be the same > > If the sort in the allocator is stable and resolves ties in free space > by using the device ID number, the above properties should guarantee > that the allocation is stable, so each new block group will have the > same functional chunk on the same device, and you get your wish. > > It's been a few months since I looked at that code, but I don't > recall seeing anything directly contradictory to the above > assumptions. > > Of course, if you have an odd number of devices, the allocator will > omit a different device on each block group, and you lose the ability > to survive (some) two-device failures. I suspect that the odds of > surviving a two-device failure are still non-zero, but less than if > you had an even number of devices. I'm not about to attempt an > ab-initio computation of the probabilities, but it shouldn't be too > hard to do either a monte-carlo simulation or a simple brute-force > enumeration of the possibilities for a given configuration. > > Hugo. > > -- > === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === > PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk > --- <Diablo-D3> My code is never released, it escapes from the --- > git repo and kills a few beta testers on the way out -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
