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);
  }

Continue reading “ceph internals : buffer lists”

Ceph uses extended file attributes to store file meta data. It is a list of key / value pairs. Some file systems implementations do not allow to store more than 2048 characters in the value associated with a key. To overcome this limitation Ceph implements chained extended attributes.
A value that is 5120 character long will be stored in three separate attributes:

• user.key : first 2048 characters
• user.key@1 : next 2048 characters
• user.key@2 : last 1024 characters

The proposed unit tests may be used as a documentation describing in detail how it is implemented from the caller point of view.
Continue reading “Chaining extended attributes in ceph”

The throttle implementation for ceph can be unit tested using threads when it needs to block. The gtest framework produces coverage information to lcov showing that 100% of the lines of code are covered.

Continue reading “unit testing ceph : the Throttle.cc example”

The ceph sources are compiled with code coverage enabled

root@ceph:/srv/ceph# ./configure --with-debug CFLAGS='-g' CXXFLAGS='-g' \
  --enable-coverage \
  --disable-silent-rules

and the tests are run

cd src ; make check-coverage

to create the HTML report which shows where tests could improve code coverage:

Continue reading “ceph code coverage (part 1/2)”