On 05/10/15 03:33, Anand Jain wrote: > > Sam, > > On 10/03/2015 11:50 PM, sam tygier wrote: >> Currently BTRFS allows you to make bad choices of data and >> metadata levels. For example -d raid1 -m raid0 means you can >> only use half your total disk space, but will loose everything >> if 1 disk fails. It should give a warning in these cases. > > Nice test case. however the way we calculate the impact of > lost device would be per chunk, as in the upcoming patch -set. > > PATCH 1/5] btrfs: Introduce a new function to check if all chunks a OK > for degraded mount > > The above patch-set should catch the bug here. Would you be able to > confirm if this patch is still needed Or apply your patch on top of > it ? > > Thanks, Anand >
If I understand the per-chunk work correctly it is to handle the case where although there are not enough disks remaining to guarantee being able to mount degraded, the arrangement of existing chunks happens to allow it (e.g. all the single chunks happen to be on a surviving disk). So while the example case in "[PATCH 0/5] Btrfs: Per-chunk degradable check", can survive a 1 disk loss, the raid levels do not guarantee survivability of a 1 disk loss after more data is written. My patch is preventing combinations of raid levels that have poor guarantees when loosing disks, but waste disk space. For example data=raid1 metadata=single, which wastes space by writing the data twice, but would not guarantee survival of a 1 disk loss (even if the per-chuck patches allow some 1 disk losses to survive) and could loose everything if a bit flip happened in a critical metadata chunk. So I think my patch is useful with or without per-chunk work. Thanks, Sam -- 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
