Each binary data record is preceded by a Record Header. The Record Header is fixed length and fixed format, whereas the binary data record is of variable length. The Record Header is written using the Bacula serialization routines and thus is guaranteed to be in machine independent format.
The format of the Record Header (version 1.27 or later) is:
int32_t FileIndex; /* File index supplied by File daemon */ int32_t Stream; /* Stream number supplied by File daemon */ uint32_t DataSize; /* size of following data record in bytes */
This record is followed by the binary Stream data of DataSize bytes, followed by another Record Header record and the binary stream data. For the definitive definition of this record, see record.h in the src/stored directory.
Additional notes on the above:
For Start Session and End Session Labels (where the FileIndex is negative), the Storage daemon uses the Stream field to contain the JobId. The current stream definitions are:
#define STREAM_UNIX_ATTRIBUTES 1 /* Generic Unix attributes */ #define STREAM_FILE_DATA 2 /* Standard uncompressed data */ #define STREAM_MD5_SIGNATURE 3 /* MD5 signature for the file */ #define STREAM_GZIP_DATA 4 /* GZip compressed file data */ /* Extended Unix attributes with Win32 Extended data. Deprecated. */ #define STREAM_UNIX_ATTRIBUTES_EX 5 /* Extended Unix attr for Win32 EX */ #define STREAM_SPARSE_DATA 6 /* Sparse data stream */ #define STREAM_SPARSE_GZIP_DATA 7 #define STREAM_PROGRAM_NAMES 8 /* program names for program data */ #define STREAM_PROGRAM_DATA 9 /* Data needing program */ #define STREAM_SHA1_SIGNATURE 10 /* SHA1 signature for the file */ #define STREAM_WIN32_DATA 11 /* Win32 BackupRead data */ #define STREAM_WIN32_GZIP_DATA 12 /* Gzipped Win32 BackupRead data */ #define STREAM_MACOS_FORK_DATA 13 /* Mac resource fork */ #define STREAM_HFSPLUS_ATTRIBUTES 14 /* Mac OS extra attributes */ #define STREAM_UNIX_ATTRIBUTES_ACCESS_ACL 15 /* Standard ACL attributes on UNIX */ #define STREAM_UNIX_ATTRIBUTES_DEFAULT_ACL 16 /* Default ACL attributes on UNIX */
The Record Header is never split across two blocks. If there is not enough room in a block for the full Record Header, the block is padded to the end with zeros and the Record Header begins in the next block. The data record, on the other hand, may be split across multiple blocks and even multiple physical volumes. When a data record is split, the second (and possibly subsequent) piece of the data is preceded by a new Record Header. Thus each piece of data is always immediately preceded by a Record Header. When reading a record, if Bacula finds only part of the data in the first record, it will automatically read the next record and concatenate the data record to form a full data record.
Kern Sibbald 2010-08-30