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To: "Martin Matuska" <mm@freebsd.org>, "Pawel Jakub Dawidek" <pjd@freebsd.org>
Subject: Re: svn commit: r243524 - in head: cddl/contrib/opensolaris/cmd/ztest
 sys/cddl/contrib/opensolaris/uts/common/fs/zfs
 sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys
References: <201211251632.qAPGW8wd091581@svn.freebsd.org>
 <20121125163924.GM1460@garage.freebsd.pl>
Date: Mon, 26 Nov 2012 11:12:26 +0100
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On Sun, 25 Nov 2012 17:39:24 +0100, Pawel Jakub Dawidek <pjd@freebsd.org>  
wrote:

> On Sun, Nov 25, 2012 at 04:32:08PM +0000, Martin Matuska wrote:
>> Author: mm
>> Date: Sun Nov 25 16:32:07 2012
>> New Revision: 243524
>> URL: http://svnweb.freebsd.org/changeset/base/243524
>>
>> Log:
>>   MFV r243013 and r243267:
>>
>>   Import the zio nop-write improvement from Illumos. To reduce I/O,
>>   nop-write omits overwriting data if the checksum (cryptographically
>>   secure) of new data matches the checksum of existing data.
>>   It also saves space if snapshots are in use.
>>
>>   It currently works only on datasets with enabled compression, disabled
>>   deduplication and sha256 checksums.
>>
>>   IllumOS 13887:196932ec9e6a and 13888:7204b3392a58
>>   3236 zio nop-write
>
> This is a nice change, although I don't understand why it depends on
> compression being turned on. It won't save any space.
>

This question is also discussed here.
http://comments.gmane.org/gmane.os.illumos.zfs/200