On 03/31/2018 09:43 AM, Zygo Blaxell wrote:
>> The key is that if a data write is interrupted, all the transaction
>> is interrupted and aborted. And due to the COW nature of btrfs, the
>> "old state" is restored at the next reboot.
> This is not presently true with raid56 and btrfs. RAID56 on btrfs uses
> RMW operations which are not COW and don't provide any data integrity
> guarantee. Old data (i.e. data from very old transactions that are not
> part of the currently written transaction) can be destroyed by this.
Could you elaborate a bit ?
Generally speaking, updating a part of a stripe require a RMW cycle, because
- you need to read all data stripe (with parity in case of a problem)
- then you should write
- the new data
- the new parity (calculated on the basis of the first read, and the
new data)
However the "old" data should be untouched; or you are saying that the "old"
data is rewritten with the same data ?
BR
G.Baroncelli
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