[sheepdog] [PATCH 7/9] Doc. "Sheepdog Advanced" add chapter "vdi read and write"

[Valerio Pachera]

Signed-off-by: Valerio Pachera <sirio81 at gmail.com>
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 doc/vdi_read_and_write.rst |   61 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 doc/vdi_read_and_write.rst

diff --git a/doc/vdi_read_and_write.rst b/doc/vdi_read_and_write.rst
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+VDI Read And Write
+==================
+
+To read a vdi means to send its content in the output standard.
+It's analogous to 'cat mydisk.img'  or 'dd if=mydisk.img'.
+You can use it to create raw images of your vdi.
+
+::
+
+    dog vdi read myvdi > /media/nas/myvdi.img
+    
+Note that all virtuall space will be read, so if you vdi size is 10G and only 
+2G are used, myvdi.img size will be 10G anyway.
+
+Viceversa, 'write' function will read from standard input end write to vdi.
+
+::
+
+    dog vdi write myvdi < /media/nas/myvdi.img
+    
+*Note:* vdi write will not create a vdi, you have to do it first.
+
+*Note 2:* you can also specifie an offset and lenght.
+You can specify the unit of misure (K,M,G)(case insensitive).
+Default is byte.
+
+**Another example:**
+
+::
+
+    dog vdi read myvdi $((2048*412)) $((204800*512)) | gzip -c > /media/nas/myvdi_sda1.img.gz
+    
+This is going to save the first partition of the vdi in a compressed file.
+This is the partition shown using sectors and MB.
+
+::
+
+    cfdisk -P t /dev/sda
+    Partition Table for /dev/sda
+
+            ---Starting----      ----Ending-----    Start     Number of
+    # Flags Head Sect  Cyl   ID  Head Sect  Cyl     Sector    Sectors
+    -- ----- ---- ---- ----- ---- ---- ---- ----- ----------- -----------
+    1  0x80   32   33     0 0x07  223   19    12        2048      204800
+    2  0x00  223   20    12 0x07  218   28  6078      206848    97449984
+    3  0x00  218   29  6078 0x8E   47   46 60801    97656832   879114240
+    4  0x00    0    0     0 0x00    0    0     0           0           0
+    
+
+                            Disk Drive: /dev/sda
+                    Size: 500107862016 bytes, 500.1 GB
+            Heads: 255   Sectors per Track: 63   Cylinders: 60801
+
+    Name     Flags      Part Type   FS Type           [Label]         Size (MB)
+    ----------------------------------------------------------------------------
+                        Pri/Log    Free Space                             1,05 *
+    sda1    Boot        Primary    ntfs        [Riservato per il]       104,86 *
+    sda2                Primary    ntfs                               49894,40 *
+    sda3                Primary    LVM2_member                       450106,50 *
+                        Pri/Log    Free Space                             1,08 *
+
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1.7.10.4