The ceph buffers are used to process data in memory. For instance, when a FileStore handles an OP_WRITE transaction it writes a list of buffers to disk.
+---------+
| +-----+ |
list ptr | | | |
+----------+ +-----+ | | | |
| append_ >-------> >--------------------> | |
| buffer | +-----+ | | | |
+----------+ ptr | | | |
| _len | list +-----+ | | | |
+----------+ +------+ ,--->+ >-----> | |
| _buffers >----> >----- +-----+ | +-----+ |
+----------+ +----^-+ \ ptr | raw |
| last_p | / `-->+-----+ | +-----+ |
+--------+-+ / + >-----> | |
| ,- ,--->+-----+ | | | |
| / ,--- | | | |
| / ,--- | | | |
+-v--+-^--+--^+-------+ | | | |
| bl | ls | p | p_off >--------------->| | |
+----+----+-----+-----+ | +-----+ |
| | off >------------->| raw |
+---------------+-----+ | |
iterator +---------+
The actual data is stored in buffer::raw opaque objects. They are accessed through a buffer::ptr. A buffer::list is a sequential list of buffer::ptr which can be used as if it was a contiguous data area although it can be spread over many buffer::raw containers, as represented by the rectangle enclosing the two buffer::raw objects in the above drawing. The buffer::list::iterator can be used to walk each character of the buffer::list as follows:
bufferlist bl;
bl.append("ABC", 3);
{
bufferlist::iterator i(&bl);
++i;
EXPECT_EQ('B', *i);
}