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Message-Id: <201107280831.p6S8VE7Y089749@red.freebsd.org>
Date: Thu, 28 Jul 2011 08:31:14 GMT
From: vermaden <vermaden@interia.pl>
To: freebsd-gnats-submit@FreeBSD.org
Subject: FEATURE REQUEST: add FLETCHER4 as DEDUP hash option
X-Send-Pr-Version: www-3.1
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>Number:         159251
>Category:       kern
>Synopsis:       [zfs] [request]: add FLETCHER4 as DEDUP hash option
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-fs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:  
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Thu Jul 28 08:40:06 UTC 2011
>Closed-Date:    
>Last-Modified:  Fri Jul 29 05:55:05 UTC 2011
>Originator:     vermaden
>Release:        8.2-STABLE
>Organization:
>Environment:
>Description:
Hi,

it would be nice to have fletcher4 as a faster alternative to sha256:

# zfs set dedup=fletcher4,verify tank
cannot set property for 'tank': 'dedup' must be one of 'on | off | verify | sha256[,verify]'

FROM: http://blogs.oracle.com/bonwick/entry/zfs_dedup
----------------------------------------------------------------------
Given the ability to detect hash collisions as described above, it is
possible to use much weaker (but faster) hash functions in combination
with the 'verify' option to provide faster dedup. ZFS offers this option
for the fletcher4 checksum, which is quite fast: 

# zfs set dedup=fletcher4,verify tank 

The tradeoff is that unlike SHA256, fletcher4 is not a pseudo-random
hash function, and therefore cannot be trusted not to collide. It is
therefore only suitable for dedup when combined with the 'verify' option,
which detects and resolves hash collisions. On systems with a very high
data ingest rate of largely duplicate data, this may provide better
overall performance than a secure hash without collision verification.
----------------------------------------------------------------------

Regards,
vermaden

>How-To-Repeat:

>Fix:


>Release-Note:
>Audit-Trail:
Responsible-Changed-From-To: freebsd-bugs->freebsd-fs 
Responsible-Changed-By: linimon 
Responsible-Changed-When: Fri Jul 29 05:54:25 UTC 2011 
Responsible-Changed-Why:  
Over to maintainer(s). 

http://www.freebsd.org/cgi/query-pr.cgi?pr=159251 
>Unformatted:
