Re: [coreutils] [PATCH] doc: show how to shred using a single zero-writing pass

[Jim Meyering]

Jim Meyering wrote:
...
> Odd... no difference in CPU time or syscall counts.
>
> I wonder if the SSD is doing something special with blocks of all zeros,
> which is reminiscent of The Reg's article:
> http://www.theregister.co.uk/2011/01/14/ocz_and_ddrdrive_performance_row/

I've redone the doc change accordingly:

>From 0f9cf6b545aa07d552538735182007ae240d6071 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Mon, 17 Jan 2011 11:32:35 +0100
Subject: [PATCH] doc: show how to shred more efficiently

* doc/coreutils.texi (shred invocation): Give an example showing how
to invoke shred in single-pass mode, and warn that -n0 --zero may
be inadequate.
---
 doc/coreutils.texi |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 9c3e2ed..8a1b3b6 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -8892,6 +8892,20 @@ shred invocation
 shred --verbose /dev/sda5
 @end example

+On modern disks, a single pass should be adequate,
+and it will take one third the time of the default three-pass approach.
+
+@example
+# 1 pass, write pseudo-random data; 3x faster than the default
+shred --verbose -n1 /dev/sda5
+@end example
+
+To be on the safe side, use at least one pass that overwrites using
+pseudo-random data.  I.e., don't be tempted to use @samp{-n0 --zero},
+in case some disk controller optimizes the process of writing blocks
+of all zeros, and thereby does not clear all bytes in a block.
+Some SSDs may do just that.
+
 A @var{file} of @samp{-} denotes standard output.
 The intended use of this is to shred a removed temporary file.
 For example:
--
1.7.3.5