On Tue, Jun 24, 2014 at 2:34 PM, Kai Krakow <[email protected]> wrote: > I'm not sure if multiple partitions can share the same cache device > partition but more or less that's it: Initialize bcache, then attach your > backing devices, then add those bcache devices to your btrfs.
Ah, if you are stuck with one bcache partition per cached device then that will be fairly painful to manage. > Yes, it will write double the data to the cache then - but only if btrfs > also did actually read both copies (which it probably does not because it > has checksums and does not need to compare data, and lets just ignore the > case that another process could try to read the same data from the other > raid member later, that case should become optimized-out by the OS cache). I didn't realize you were proposing read caching only. If you're only caching reads then obviously that is much safer. I think with btrfs in raid1 mode with only two devices you can tell it to prefer a particular device for reading in which case you could just bcache that drive. It would only read from the other drive if the cache failed. However, I don't think btrfs lets you manually arrange drives into array-like structures. It auto-balances everything which is usually a plus, but if you have 30 disks you can't tell it to treat them as 6x 5-disk RAID5s vs one 30-disk raid5 (I think). Rich
