Roch wrote:
Bob Friesenhahn writes:
> On Wed, 23 Sep 2009, Ray Clark wrote:
>
> > My understanding is that if I "zfs set checksum=<different>" to
> > change the algorithm that this will change the checksum algorithm
> > for all FUTURE data blocks written, but does not in any way change
> > the checksum for previously written data blocks.
>
> This is correct. The same applies to blocksize and compression.
>
With an important distinction. For compression,checksum a
block rewrite will affect the next update to any fileblock.
For the dataset recordsize property, a block rewrite on an
existing multiblock file will not change the file's block
size. For Multi-record file's, the recordsize is immutable
and dissociated from the dataset recordsize setting.
> > I need to corroborate this understanding. Could someone please
> > point me to a document that states this? I have searched and
> > searched and cannot find this.
>
Me neither, although it's easy to verify that setting the
checksum property on a dataset does not induce the I/O that
would be required for a rewrite of the bp.
It is mentioned in zfs(1) for the copies property but not for checksum
and compression:
Changing this property only affects newly-written data.
Therefore, set this property at file system creation
time by using the -o copies=N option.
I've filed a man page bug 6885203 to have similar text added for
checksum and compression.
--
Darren J Moffat
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