Date: Wed, 27 Apr 2005 18:43:29 -0400 (EDT) From: Daniel Ellard <ellard@eecs.harvard.edu> To: freebsd-fs@freebsd.org Cc: Robert Krten <root@parse.com> Subject: Re: Background block scrubbing Message-ID: <20050427183037.E46204@bowser.eecs.harvard.edu> In-Reply-To: <867jin2a2p.fsf@xps.des.no> References: <200504271918.PAA05842@parse.com> <867jin2a2p.fsf@xps.des.no>
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On Wed, 27 Apr 2005, [iso-8859-1] Dag-Erling Sm=F8rgrav wrote: > "Robert Krten" <root@parse.com> writes: > > Is there a utility that does background unused block scrubbing? > > > > What I'm thinking of is something that looks for unused blocks on the > > disk, and then writes zeros, then random, then more random, etc, to the= m > > for security applications. > > That's not how it's done. Here's a good explanation of how to do it > and why it must be done that way: > > http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html That's not how it's done *now*, but it could be done like that in the future. The problem with this approach is that it might take an unknown amount of time before a particular block is sufficiently scrubbed. If the user needs to have something scrubbed right away (or by any given time) then this won't work -- but if the user has relaxed time constraints and a good amount of idle time, this doesn't seem like a bad way to get started on the process. Another suggestion -- look at the work done on free block scheduling, which opportunistically schedules I/Os for "free" during time that would ordinarily be rotational delays. There was a study a few years ago that showed that a free block scheduler could read or write every block on a disk in a matter of hours, even if the disk was heavily utilized during this period -- no need to wait for the disk to be idle. [Note that this scheduleing is "free" in terms of time, but not power or wear -- if you've got a disk that's designed to be idle a good fraction of the time, like most PC disks, this could cook it.] -Dan
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