File: ddrescue.info,  Node: Direct disc access,  Next: Command mode,  Prev: Examples,  Up: Top

11 Direct disc access
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If you notice that the positions and sizes in MAPFILE are always multiples
of the sector size, maybe your kernel is caching the disc accesses and
grouping them. In this case you may want to use direct disc access for
INFILE, or read from a raw device, to bypass the kernel cache and rescue
more of your data.

   NOTE! Sector size must be correctly set with the option '--sector-size'
for direct disc access to work.

   NOTE: Direct disc access can copy arbitrary domains by reading whole
sectors and then writing only the requested part. This is the only case
where ddrescue tries to read data outside the rescue domain.

   Try the option '--idirect' first. If direct disc access is not available
in your system, try raw devices. Read your system documentation to find how
to bind a raw device to a regular block device. Some OSs provide raw access
through especial device names, like '/dev/rdisk'.

   Ddrescue aligns its I/O buffer to the sector size so that it can be used
for direct disc access or to read from raw devices. For efficiency reasons,
also aligns it to the memory page size if page size is a multiple of sector
size. On some systems, ddrescue can't determine the size of a raw device,
so an explicit '--size' or '--complete-only' option may be needed.

   Using direct disc access, or reading from a raw device, may be slower or
faster than normal cached reading depending on your OS and hardware. In case
it is slower you may want to make a first pass using normal cached reads and
use direct disc access, or a raw device, only to recover the good sectors
inside the failed blocks.


Example 1: using direct disc access.

     ddrescue -f -n /dev/sdb1 /dev/sdc1 mapfile
     ddrescue -d -f -r3 /dev/sdb1 /dev/sdc1 mapfile
     e2fsck -v -f /dev/sdc1
     mount -t ext2 -o ro /dev/sdc1 /mnt


Example 2: using a raw device.

     raw /dev/raw/raw1 /dev/sdb1
     ddrescue -f -n /dev/sdb1 /dev/sdc1 mapfile
     ddrescue -C -f -r3 /dev/raw/raw1 /dev/sdc1 mapfile
     raw /dev/raw/raw1 0 0
     e2fsck -v -f /dev/sdc1
     mount -t ext2 -o ro /dev/sdc1 /mnt