From 74b43f0630cd644059d09d38aa10bd123100d4f9 Mon Sep 17 00:00:00 2001
From: Jan Sucan <sucanjan@fit.cvut.cz>
Date: Fri, 12 Apr 2019 08:01:58 +0200
Subject: Add README.md

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+# diff-dd
+
+This simple utility was created to reduce size of backup images of
+disk partitions. It is intended to be used in conjunction with ```dd
+``` or similar utility. The primary concern is safe operation so
+```diff-dd``` performs many checks of the input and output
+files. Because of the it is slower or less effective,
+e.g. differential image file is read twice when restoring it.
+
+## Synopsis
+
+> diff-dd [-s SECTOR_SIZE] [-b BUFFER_SIZE] [INFILE]  REFFILE  OUTFILE
+
+## Usage
+
+The utility is used for creating and restoring differential images
+created by it.  Meaning of the ```INFILE``` and ```REFFILE``` depends
+on whether backup mode or restore mode is requested. Providing ```INFILE```
+selects the backup mode. Omitting it selects the restore mode.
+
+## Backup
+
+Using ```diff-dd ``` for backup requires the full backup image to
+exist. Differential backup is created with:
+
+> diff-dd INFILE REFFILE OUTFILE
+
+The ```INFILE``` is a path to the file to backup differentially, the
+```REFFILE``` is the full image, and the ```OUTFILE``` is the file to
+which only the changed sectors of the ```INFILE```, compared to the
+```REFFILE```, and their offsets will be saved.
+
+## Restore
+
+The restoration means application of the changed sectors saved in the
+```REFFILE```, which is the differential image, to the ```OUTFILE```:
+
+> diff-dd REFFILE OUTFILE
+
+## Options
+
+```-s``` sets the sector size by which the files will be processed
+(default is 512 B). In can be used to control granularity of
+differential backup.
+
+```-b``` sets the size of the buffer for the sectors of the input and
+output file (default is 4 MiB). The input data is always buffered. The
+output data are buffered only in backup mode.
+
+## Example
+
+First, the full image of the partition to backup has to be created:
+
+> dd bs=4M if=/dev/sda1 of=full.img
+
+When user decides to create the differential image, he or she runs:
+
+> diff-dd /dev/sda1 full.img diff.img
+
+If a data accident happens, the partition will be restored by running:
+
+> dd bs=4M if=full.img of=/dev/sda1
+> diff-dd diff.dd /dev/sda1
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