Date: Sun, 30 Mar 2008 20:31:23 +0000 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Kirk McKusick <mckusick@mckusick.com>, arch@freebsd.org Subject: Re: Flash disks and FFS layout heuristics Message-ID: <12858.1206909083@critter.freebsd.dk> In-Reply-To: Your message of "Sun, 30 Mar 2008 13:16:56 MST." <200803302016.m2UKGuZA015127@apollo.backplane.com>
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In message <200803302016.m2UKGuZA015127@apollo.backplane.com>, Matthew Dillon w rites: > The right way to deal with flash is *NOT* to require that the filesystem > be smart about flash storage, but instead to implement an intermediate > storage layer which linearizes the writes to flash and removes all > random erases from the critical path. Your description of a simplified version of what is commonly called a "Flash Adaptation Layer", is a very good example of why there is a clear difference between "camera grade" flash devices, like most CF cards, and the new generation of "SSD" devices, like the M-Tron disk now in my laptop. The Camera grade Flash devices get lousy random write performance because they implement in essense what you describe, only in a more complete fashion where they have error correction, both the data and on the bitmaps. The newer generation of SSD devices do things much smarter than that, which is why their random write performance is much better than camera-grade devices. See my earlier emails for references to how to do the really smart thing. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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