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-* How does file translates to key/value pair?
- in bdb a file is identified by key (obtained by taking basename() of the path of
-the file) and file contents are stored as value corresponding to the key in database
-file (defaults to glusterfs_storage.db under dirname() directory).
-* symlinks, directories
- symlinks and directories are stored as is.
-* db (database) files
- every directory, including root directory, contains a database file called
-glusterfs_storage.db. all the regular files contained in the directory are stored
-as key/value pair inside the glusterfs_storage.db.
-* internal data cache
- db does not provide a way to find out the size of the value corresponding to a key.
-so, bdb makes DB->get() call for key and takes the length of the value returned.
-since DB->get() also returns file contents for key, bdb maintains an internal cache and
-stores the file contents in the cache.
- every directory maintains a seperate cache.
-* inode number transformation
- bdb allocates a inode number to each file and directory on its own. bdb maintains a
-global counter and increments it after allocating inode number for each file
-(regular, symlink or directory). NOTE: bdb does not guarantee persistent inode numbers.
-* checkpoint thread
- bdb creates a checkpoint thread at the time of init(). checkpoint thread does a
-periodic checkpoint on the DB_ENV. checkpoint is the mechanism, provided by db, to
-forcefully commit the logged transactions to the storage.
-lookup() -
- 1> do lstat() on the path, if lstat fails, we assume that the file being looked up
- is either a regular file or doesn't exist.
- 2> lookup in the DB of parent directory for key corresponding to path. if key exists,
- return key, with.
- NOTE: 'struct stat' stat()ed from DB file is used as a container for 'struct stat'
- of the regular file. st_ino, st_size, st_blocks are updated with file's values.
-readv() -
- 1> do a lookup in bctx cache. if successful, return the requested data from cache.
- 2> if cache missed, do a DB->get() the entire file content and insert to cache.
- 1> flush any cached content of this file.
- 2> do a DB->put(), with DB_DBT_PARTIAL flag.
- NOTE: DB_DBT_PARTIAL is used to do partial update of a value in DB.
- 1> regular readdir() in a loop, and vomit all DB_ENV log files and DB files that
- we encounter.
- 2> if the readdir() buffer still has space, open a DB cursor and do a sequential
- DBC->get() to fill the reaadir buffer.