Date: Fri, 18 Dec 2009 19:55:30 +0100 From: Bernd Walter <ticso@cicely7.cicely.de> To: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org> Cc: freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: ZFS RaidZ2 with 24 drives? Message-ID: <20091218185529.GC1531@cicely7.cicely.de> In-Reply-To: <4B2B9F82.4020909@jrv.org> References: <568624531.20091215163420@pyro.de> <42952D86-6B4D-49A3-8E4F-7A1A53A954C2@spry.com> <957649379.20091216005253@pyro.de> <26F8D203-A923-47D3-9935-BE4BC6DA09B7@corp.spry.com> <deb820500912161320n43c552d7rf84264332574a701@mail.gmail.com> <E9C46E04-1A81-4EE5-909E-557EA08D16A9@corp.spry.com> <4B299CEA.3070705@jrv.org> <deb820500912161908o7736a667qe24eec9e33f53a8@mail.gmail.com> <4B2B9F82.4020909@jrv.org>
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On Fri, Dec 18, 2009 at 09:28:02AM -0600, James R. Van Artsdalen wrote: > Thomas Burgess wrote: > > One thing most people don't know about hard drives in general is that > > sometimes up to 30% of the space is actually ECC. With software raid > > systems like ZFS, this will eventually be somethign that we can take > > advantage of. > > ECC is less than 10% of the space. The inter-sector gap and gap between > a sector's address and data fields, etc, are larger and more problematic > as rotation speeds increase. > > > Because of this, you can imagine a scenario where allowing ZFS to > > use this ECC space as raw storage, while leaving the data corrections > > to ZFS would be ideal. It's not only a matter of space, it will also > > lead to nice improvements in speed. (more data can be read/written by > > the head as it passes) I can imagine this might work, but do not see it to be very realistic. And I'm not sure this is a very good idea as well. > The disk drive industry's solution to this is 4K sector sizes. See > http://www.anandtech.com/storage/showdoc.aspx?i=3691 This is an quite interesting article, but for me not very surprising. It is a bit more surprising that this is not already standard. The whole thing is not new - the Commodore 1581 floppy drive did a track at once logic with caching and supplied logical 256 Byte sectors with underlying 512 MFM blocks. With flash cards 4k physical sectors are also very common and writing smaller/unaligned transfers leads to slow read-modify-write cycles. You need to be very carefull when partitioning a flash based memory card - a lot of us are already using flash based as an alternative to an HDD. People also used MO media with 1k and 2k hard sectoring - they usually don't have an emulation for 512 Bytes. -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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