Re: Questions on use of NOCOW impact to subvolumes and snapshots

[Austin S Hemmelgarn]

On 2015-08-20 00:40, Jonathan Panozzo wrote:

Zhao,

Thank you for your response.  Two quick follow-up questions:

1:  What happens on an unrecoverable data error case?  Does the volume get put 
into read-only mode?

Yes.

2:  Out of curiosity, why is data checksumming tied to COW?

There's no safe way to sanely handle checksumming without COW, because there is no way (at least on current hardware) to ensure that the data block and the checksums both get written at the exact same time, and that one of the writes aborting will cause the other too do so as well. In-place compression is disabled for nodatasum files for essentially the same reason (although compression can cause much worse data loss than a failed checksum).

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