Date: Sat, 14 Aug 2004 05:45:08 +0100 From: David Kreil <kreil@ebi.ac.uk> To: Brooks Davis <brooks@one-eyed-alien.net> Cc: David Kreil <kreil@ebi.ac.uk> Subject: Re: "sanitizing" disks: wiping swap, non-allocated space, and file-tails Message-ID: <200408140445.i7E4j8001670@puffin.ebi.ac.uk> In-Reply-To: Your message of "Tue, 20 Jul 2004 15:00:33 PDT." <20040720220033.GA12560@Odin.AC.HMC.Edu>
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Dear Brooks,
> > > > > The easiest way to scrub a disk is:
> > > > >
> > > > > dd if=/dev/random of=/dev/<disk> bs=<sthg big>
> > > > > <repeat a few times>
> >
> > I noticed that it will refuse to let me do that on swap, even if it is
> > of f. Of course, I can edit the disklabel to read "unused", run dd, and
> > restore the swap disklabel to "swap" but is there another way?
>
> That's broken. Which OS are you using?
Don't know whether I answered that before: 5.2.1-RELEASE-p9/GENERIC
To which list, if not fs, should I send a bug-report in your opinion?
> > Also, I've just done some tests, and
> >
> > dd if=/dev/random of=/dev/<mydisk> bs=1048576
> >
> > only writes at 6.5MB/s on my system (/dev/zero gives 7.9MB/s). Is that=20
> > typical? My drives theoretically should do 30-40MB/s on read, and
> > 20-30MB/s on write.
> >
> > If these results are "normal", however, that means, for a 10GB swap file
> > and, say 6 wipes, I'd be waiting 3h on shutdown, while a BND-safe thorough
> > 20 wipes would take half a day. Not really practical :-/
> > So unless you tell me that I should be able to achieve much faster write
> > speeds, I think I'll have to ditch the idea of regularly wiping swap (or
> > anything else for that matter).
Actually, I just had one of the drives in my RAID replaced (which was
apparently on its way breaking down) and now get ~50MB/s write performance for
dd if=/dev/zero, and ~13MB/s for /dev/random. So if I could generate good
pseudo-random numbers fast enough, I should be able to wipe a 10GB partition
20x in an hour - that's good enough!
> If you
> really want performance, you should use arc4random in a custom userland
> program. That's faster, but expect wiping a 40GB disk to take hours
> even in that case. I've got such an application, but I haven't had time
> to clean it up and submit it for release. I'll probably do it some day,
> but I can't recommend waiting for that. It's only about 800 lines of
> code including the man page and a fancy composable operations system to
> allow just about any DoD or non-DoD pattern or writes and verifies to be
> written on the command line.
I'd be grateful if you could make your utility available. All I need is random
patterns (white noise). Would that be possible at all, please?
With best regards,
David.
------------------------------------------------------------------------
Dr David Philip Kreil ("`-''-/").___..--''"`-._
Research Fellow `6_ 6 ) `-. ( ).`-.__.`)
University of Cambridge (_Y_.)' ._ ) `._ `. ``-..-'
++44 1223 764107, fax 333992 _..`--'_..-_/ /--'_.' ,'
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