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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.
NOTES ABOUT FOPS:
-----------------
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.
writev():
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.
readdir():
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.
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