[zfs-discuss] COW when re-writing identical data?

Richard Laager rlaager at wiktel.com
Fri Dec 2 21:46:48 EST 2016

On 12/02/2016 06:57 PM, Nikolaus Rath via zfs-discuss wrote:
> If I override a file with the same data, will ZFS recognize that the
> data hasn't changed and not create a copy?

ZFS has a feature called nopwrite. ZFS will not replace a block with the
same contents if:

1) The old and new blocks are checksummed using the same algorithm.
2) That algorithm is cryptographically secure (e.g. sha256)
3) Compression is enabled on that block.
" -- https://www.illumos.org/issues/3236

I don't know that it's been released yet, but ZoL 0.7.0 should support
checkum=edonr, checksum=skein, and checksum=sha512. These are all faster
than sha256 (and I've listed them in order of decreasing performance).

So for now, try with compression=lz4 & checksum=sha256. Later, you could
use compression=lz4 and checksum=edonr.


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