From owner-freebsd-current@FreeBSD.ORG Wed Aug 24 19:24:52 2011 Return-Path: Delivered-To: current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 4F536106566B; Wed, 24 Aug 2011 19:24:52 +0000 (UTC) (envelope-from kaduk@mit.edu) Received: from dmz-mailsec-scanner-7.mit.edu (DMZ-MAILSEC-SCANNER-7.MIT.EDU [18.7.68.36]) by mx1.freebsd.org (Postfix) with ESMTP id CE6658FC14; Wed, 24 Aug 2011 19:24:51 +0000 (UTC) X-AuditID: 12074424-b7bcaae000000a05-f4-4e55502ec082 Received: from mailhub-auth-4.mit.edu ( [18.7.62.39]) by dmz-mailsec-scanner-7.mit.edu (Symantec Messaging Gateway) with SMTP id 4C.51.02565.E20555E4; Wed, 24 Aug 2011 15:25:34 -0400 (EDT) Received: from outgoing.mit.edu (OUTGOING-AUTH.MIT.EDU [18.7.22.103]) by mailhub-auth-4.mit.edu (8.13.8/8.9.2) with ESMTP id p7OJOoL8015751; Wed, 24 Aug 2011 15:24:51 -0400 Received: from multics.mit.edu (MULTICS.MIT.EDU [18.187.1.73]) (authenticated bits=56) (User authenticated as kaduk@ATHENA.MIT.EDU) by outgoing.mit.edu (8.13.6/8.12.4) with ESMTP id p7OJOn6j012042 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT); Wed, 24 Aug 2011 15:24:50 -0400 (EDT) Received: (from kaduk@localhost) by multics.mit.edu (8.12.9.20060308) id p7OJOmMm024903; Wed, 24 Aug 2011 15:24:48 -0400 (EDT) Date: Wed, 24 Aug 2011 15:24:48 -0400 (EDT) From: Benjamin Kaduk To: Hiroki Sato In-Reply-To: <20110825.041315.156163335542976067.hrs@allbsd.org> Message-ID: References: <20110824082119.GJ17489@deviant.kiev.zoral.com.ua> <920337541.272757.1314192294772.JavaMail.root@erie.cs.uoguelph.ca> <20110825.041315.156163335542976067.hrs@allbsd.org> User-Agent: Alpine 1.10 (GSO 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; format=flowed; charset=US-ASCII X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFmpkleLIzCtJLcpLzFFi42IRYrdT19ULCPUzuPKAz2LClR9MFrfaVzFb NEx7zGZxeepSNouHy64xObB6zPg0n8Vj56y77B6/N+9lCmCO4rJJSc3JLEst0rdL4Mq4uDWo 4C1bxfIXm1kbGDezdjFyckgImEjcfryNGcIWk7hwbz1bFyMXh5DAPkaJNSeOs0A4GxglFvW3 MUI4B5gk5t7+zAzhNDBK7Nx1nBGkn0VAW2LqkiNgc9kEVCRmvtkINIuDQ0RAUaL/iDdImFkg RuLp0g9sILawgILEhLvrmEBsTgEHiZ6HU8HivED23P/vmCDmb2WUePRrOdh8UQEdidX7p7BA FAlKnJz5hAViqKXEv7W/WCcwCs5CkpqFJLWAkWkVo2xKbpVubmJmTnFqsm5xcmJeXmqRrrle bmaJXmpK6SZGcEi7qOxgbD6kdIhRgINRiYf3gkOonxBrYllxZe4hRkkOJiVR3jd+QCG+pPyU yozE4oz4otKc1OJDjBIczEoivKfOhfgJ8aYkVlalFuXDpKQ5WJTEeW12OvgJCaQnlqRmp6YW pBbBZGU4OJQkePX8gYYKFqWmp1akZeaUIKSZODhBhvMADX8Aspi3uCAxtzgzHSJ/ilFRSpzX FaRZACSRUZoH1wtLOa8YxYFeEeaNBaniAaYruO5XQIOZgAZz/AoCGVySiJCSamCsU1T8svHV R4H0JRd6nhq2lNywldyW8VzN70bXOh1RzWL1ix6t9eeWFd821X75tbE+3nbZlqca9c+mrdi7 6sMd/ZVGjxYf0XRm2N43S2LDtELnsnMMpvvEYie/uJQXqXF3a8OVlQeEz2k+nrJp9cOsrZFz FcUEerQ3pERMnVi6kCtBPP9YymspJZbijERDLeai4kQAdUpSFhQDAAA= Cc: kostikbel@gmail.com, rmacklem@uoguelph.ca, pjd@freebsd.org, current@freebsd.org Subject: Re: fsid change of ZFS? X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 24 Aug 2011 19:24:52 -0000 On Thu, 25 Aug 2011, Hiroki Sato wrote: > > My opinion is using a hash function which occurs no collision in the > well-known names is sufficient for our purpose because the number of > file systems on a running system is small anyway and not changed > frequently in most cases. I am not sure which function is best; > fnv_32_str() seemed better against a large data set but it is not > likely to have >30 file systems, for example. This was a large part of my reasoning when proposing a hash table initially -- there are 256 (maybe 255 if you want to reserve one) slots, and only 14 or so filesystems currently in the tree. It seems very unlikely that any single machine would ever have more than another five out-of-tree filesystem types on it (right?), so putting 20 items into 250 hash bins is a pretty good chance of being collision-free. -Ben