Date: Sun, 16 Oct 2011 16:13:26 -0500 (CDT) From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> To: Jeremy Chadwick <freebsd@jdc.parodius.com> Cc: freebsd-fs@freebsd.org Subject: Re: [ZFS] Using SSD with partitions Message-ID: <alpine.GSO.2.01.1110161607120.4501@freddy.simplesystems.org> In-Reply-To: <20111016183003.GA29466@icarus.home.lan> References: <CACh33Fpz=uAp8h0Bjsi1Be=ob_94jXtN51mAHvGPkReY5MpTcg@mail.gmail.com> <4E9AE725.4040001@gmail.com> <169E82FD-3B61-4CAB-B067-D380D69CDED5@digsys.bg> <4E9B1C1E.7090804@gmail.com> <20111016183003.GA29466@icarus.home.lan>
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On Sun, 16 Oct 2011, Jeremy Chadwick wrote: > > 2) I would like an explanation as to what "SSDs are more likely than an > MHDD to lose data on a power outage" means exactly (on a technical > level, not something vague) and from where you got this interpretation. The reason is that normal operation of the SSD will move and/or rewrite existing data, which is also likely to be much older than the data currently being written. Common reasons are wear leveling, garbage collection (compacting) and because the block written is not identically sized and aligned with the SSDs native underlying blocks. While data is being re-written, moved, or copied, a copy resides in RAM. A SSD which is more defensive about avoiding corrupting old data is also likely to be slower to synchronously write. There are certainly algorithms (e.g. as used by zfs) which can help an SSD avoid issues. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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