Re: Status of RAID5/6

[Goffredo Baroncelli <kreijack@inwind.it>]

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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