DbMpoolFile
#include <db_cxx.h>
static int
DbMpoolFile::open(DbMpool *mp, char *file, u_int32_t flags, int mode,
size_t pagesize, DbMpoolFinfo *finfop, DbMpoolFile **mpf);
int
DbMpoolFile::close();
int
DbMpoolFile::get(db_pgno_t *pgnoaddr, u_int32_t flags, void **pagep);
int
DbMpoolFile::put(void *pgaddr, u_int32_t flags);
int
DbMpoolFile::set(void *pgaddr, u_int32_t flags);
int
DbMpoolFile::sync();
DESCRIPTION
The DB library is a family of classes that provides a
modular programming interface to transactions and record-
oriented file access. The library includes support for
transactions, locking, logging and file page caching, as
well as various indexed access methods. Many of the
classes (e.g., the file page caching class) are useful
independent of the other DB classes, although some classes
are explicitly based on other classes (e.g., transactions
and logging). For a general description of the DB
package, see db_intro(3).
This manual page describes the specific details of the
per-file memory pool interface.
The DbMpool(3) and DbMpoolFile(3) classes are the library
interface intended to provide general-purpose, page-
oriented buffer management of one or more files. While
designed to work with the other Db functions, these
functions are also useful for more general purposes. The
memory pools (DbMpool::'s) are referred to in this
document as simply ``pools''. Pools may be shared between
processes. Pools are usually filled by pages from one or
more files (DbMpoolFile's). Pages in the pool are
replaced in LRU (least-recently-used) order, with each new
page replacing the page that has been unused the longest.
Pages retrieved from the pool using DbMpoolFile::get are
``pinned'' in the pool, by default, until they are
returned to the pool's control using the DbMpoolFile::put
method.
DbMpoolFile::open
The DbMpoolFile::open method opens a file in the pool
specified by the DbMpool argument, copying the DbMpoolFile
pointer representing it into the memory location
referenced by mpf.
The file argument is the name of the file to be opened.
If file is NULL, a private file is created that cannot be
shared with any other process (although it may be shared
with other threads).
The flags and mode arguments specify how files will be
opened and/or created when they don't already exist. The
flags value is specified by or'ing together one or more of
the following values:
DB_CREATE
Create any underlying files, as necessary. If the
files do not already exist and the DB_CREATE flag is
not specified, the call will fail.
DB_NOMMAP
Always copy this file into the local cache instead of
mapping it into process memory (see the description
of the mp_mmapsize field of the DbEnv object for
further information).
DB_RDONLY
Open any underlying files for reading only. Any
attempt to write the file using the pool functions
will fail, regardless of the actual permissions of
the file.
All files created by the method DbMpoolFile::open are
created with mode mode (as described in chmod(2)) and
modified by the process' umask value at the time of
creation (see umask(2)). The group ownership of created
files is based on the system and directory defaults, and
is not further specified by DB.
The pagesize argument is the size, in bytes, of the unit
of transfer between the application and the pool, although
it is not necessarily the unit of transfer between the
pool and the source file.
Files opened in the pool may be further configured based
on the finfop argument to memp_fopen, which is a pointer
to a structure of type DB_MPOOL_FINFO (typedef'd in
<db.h>). No references to the finfop structure are
maintained by DB, so it may be discarded when the
memp_fopen function returns. In order to ensure
compatibility with future releases of DB, all fields of
the DB_MPOOL_FINFO structure that are not explicitly set
should be initialized to 0 before the first time the
structure is used. Do this by declaring the structure
external or static, or by calling the C library routine
bzero(3) or memset(3).
The fields of the DB_MPOOL_FINFO structure used by
DbMpoolFile::open are described below. If finfop is NULL
or any of its fields are set to their default value,
defaults appropriate for the system are used.
int ftype;
The ftype field should be the same as a ftype
argument previously specified to the
DbMpool::db_register method, unless no input or
output processing of the file's pages are necessary,
in which case it should be 0. (See the description
of the DbMpool::db_register method for more
information.)
DBT *pgcookie;
The pgcookie argument contains the byte string that
is passed to the pgin and pgout functions for this
file, if any. If no pgin or pgout functions are
specified, the pgcookie field should be NULL. (See
the description of the DbMpool::db_register method
for more information.)
u_int8_t *fileid;
The fileid field is a unique identifier for the file.
The mpool functions must be able to uniquely identify
files in order that multiple processes sharing a file
will correctly share its underlying pages. Normally,
the fileid field should be NULL and the mpool
functions will use the file's device and inode
numbers (see stat(2)) for this purpose. On some
filesystems, (e.g., FAT or NFS) file device and inode
numbers are not necessarily unique across system
reboots. Applications wanting to maintain a shared
memory buffer pool across system reboots, where the
pool contains pages from files stored on such
filesystems, must specify a unique file identifier to
the DbMpoolFile::open call and each process opening
or registering the file must provide the same unique
identifier. If the fileid field is non-NULL, it must
reference a DB_FILE_ID_LEN (as defined in <db_cxx.h>)
length array of bytes that will be used to uniquely
identify the file. This should not be necessary for
most applications. Specifically, it is not necessary
if the memory pool is re-instantiated after each
system reboot, the application is using the Db access
methods instead of calling the pool functions
explicitly, or the files in the memory pool are
stored on filesystems where the file device and inode
numbers do not change across system reboots.
int32_t lsn_offset;
The lsn_offset argument is the zero-based byte offset
in the page of the page's log sequence number (LSN),
or -1 if no LSN offset is specified. (See the
description of the DbMpool::sync method for more
information.)
u_int32_t clear_len;
The clear_len field is the number of initial bytes in
a page that should be set to zero when the page is
created as a result of the DB_MPOOL_CREATE or
DB_MPOOL_NEW flags being specified to
DbMpoolFile::get. If finfop is NULL or clear_len is
0, the entire page is cleared.
The DbMpoolFile::open method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbMpoolFile::close
The DbMpoolFile::close method closes the source file
indicated by the DbMpoolFile object. This method does not
imply a call to DbMpoolFile::sync, i.e. no pages are
written to the source file as as a result of calling
DbMpoolFile::close.
In addition, if the file argument to DbMpoolFile::open was
NULL, any underlying files created for this DbMpoolFile
will be removed.
The DbMpoolFile::close method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbMpoolFile::get
The DbMpoolFile::get method copies a pointer to the page
with the page number specified by pgnoaddr, from the
source file specified by the DbMpoolFile object into the
memory location referenced by pagep. If the page does not
exist or cannot be retrieved, DbMpoolFile::get will fail.
The returned page is size_t type aligned.
Page numbers begin at 0, e.g., the first page in the file
is page number 0, not page number 1.
The flags argument is specified by or'ing together one or
more of the following values:
DB_MPOOL_CREATE
If the specified page does not exist, create it. In
this case, the pgin method, if specified, is called.
DB_MPOOL_LAST
Return the last page of the source file and copy its
page number to the location referenced by pgnoaddr.
DB_MPOOL_NEW
Create a new page in the file and copy its page
number to the location referenced by pgnoaddr. In
this case, the pgin method, if specified, is not
called.
The DB_MPOOL_CREATE, DB_MPOOL_LAST and DB_MPOOL_NEW flags
are mutually exclusive.
Created pages have all their bytes set to 0, unless
otherwise specified when the file was opened.
All pages returned by DbMpoolFile::get will be retained
(i.e. ``pinned'') in the pool until a subsequent call to
DbMpoolFile::put.
The DbMpoolFile::get method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbMpoolFile::put
The DbMpoolFile::put method indicates that the page
referenced by pgaddr can be evicted from the pool. Pgaddr
must be an address previously returned by
DbMpoolFile::get.
The flags argument is specified by or'ing together one or
more of the following values:
DB_MPOOL_CLEAN
Clear any previously set modification information
(i.e., don't bother writing the page back to the
source file).
DB_MPOOL_DIRTY
The page has been modified and must be written to the
source file before being evicted from the pool.
DB_MPOOL_DISCARD
The page is unlikely to be useful in the near future,
and should be discarded before other pages in the
pool.
The DB_MPOOL_CLEAN and DB_MPOOL_DIRTY flags are mutually
exclusive.
The DbMpoolFile::put method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbMpoolFile::set
The DbMpoolFile::set method sets the flags associated with
the page referenced by pgaddr without unpinning it from
the pool. Pgaddr must be an address previously returned
by DbMpoolFile::get. The flags argument to
DbMpoolFile::set is specified by or'ing together one or
more of the values specified as flags for the
DbMpoolFile::put call.
The DbMpoolFile::set method throws a DbException(3) or
returns the value of errno on failure and 0 on success.
DbMpoolFile::sync
The DbMpoolFile::sync method writes all pages associated
with the DbMpoolFile object that were marked as modified
using DbMpoolFile::put or DbMpoolFile::set, back to the
source file. If any of the modified pages are also pinned
(i.e., currently referenced by this or another process)
DbMpoolFile::sync will ignore them.
The DbMpoolFile::sync method throws a DbException(3) or
returns the value of errno on failure, 0 on success, and
DB_INCOMPLETE if there were pages which were modified but
which DbMpoolFile::sync was unable to write.
ERRORS
Methods marked as returning errno will, by default, throw
an exception that encapsulates the error information. The
default error behavior can be changed, see DbException(3).
The DbMpoolFile::open method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DBmemp->pgin(3),
DBmemp->pgout(3), DbLog::compare(3), DbLog::flush(3),
close(2), fcntl(2), fflush(3), fsync(2), lseek(2),
malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2),
open(2), sigfillset(3), sigprocmask(2), stat(2),
strcpy(3), strdup(3), strerror(3), strlen(3), time(3),
unlink(2), and write(2).
In addition, the DbMpoolFile::open method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The file has already been entered into the pool, and
the pagesize value is not the same as when the file
was entered into the pool, or the length of the file
is not zero or a multiple of the pagesize.
The DB_RDONLY flag was specified for an in-memory
pool.
The DbMpoolFile::close method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: close(2), fcntl(2),
fflush(3), munmap(2), and strerror(3).
The DbMpoolFile::get method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DBmemp->pgin(3),
DBmemp->pgout(3), DbLog::compare(3), DbLog::flush(3),
close(2), fcntl(2), fflush(3), fsync(2), lseek(2),
malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2),
open(2), read(2), sigfillset(3), sigprocmask(2), stat(2),
strcpy(3), strdup(3), strerror(3), strlen(3), time(3),
unlink(2), and write(2).
In addition, the DbMpoolFile::get method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EAGAIN]
The page reference count has overflowed. (This
should never happen unless there's a bug in the
application.)
[EINVAL]
An invalid flag value or parameter was specified.
The DB_MPOOL_NEW flag was set and the source file was
not opened for writing.
The requested page does not exist and DB_MPOOL_CREATE
was not set.
More than one of DB_MPOOL_CREATE, DB_MPOOL_LAST and
DB_MPOOL_NEW was set.
[ENOMEM]
The cache is full and no more pages will fit in the
pool.
The DbMpoolFile::put method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DBmemp->pgin(3),
DBmemp->pgout(3), DbLog::compare(3), DbLog::flush(3),
close(2), fcntl(2), fflush(3), fsync(2), lseek(2),
malloc(3), memcmp(3), memcpy(3), memset(3), mmap(2),
open(2), sigfillset(3), sigprocmask(2), stat(2),
strcpy(3), strdup(3), strerror(3), strlen(3), time(3),
unlink(2), and write(2).
In addition, the DbMpoolFile::put method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EACCES]
The DB_MPOOL_DIRTY flag was set and the source file
was not opened for writing.
[EINVAL]
An invalid flag value or parameter was specified.
The pgaddr parameter does not reference a page
returned by DbMpoolFile::get.
More than one of DB_MPOOL_CLEAN and DB_MPOOL_DIRTY
was set.
The DbMpoolFile::set method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: fcntl(2), and
fflush(3).
In addition, the DbMpoolFile::set method may fail and
throw a DbException(3) or return errno for the following
conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The DbMpoolFile::sync method may fail and throw a
DbException(3)
or return errno for any of the errors specified for the
following DB and library functions: DBmemp->pgin(3),
DBmemp->pgout(3), DbLog::compare(3), DbLog::flush(3),
close(2), fcntl(2), fflush(3), fsync(2), lseek(2),
malloc(3), memcpy(3), memset(3), open(2), qsort(3),
realloc(3), sigfillset(3), sigprocmask(2), stat(2),
strcpy(3), strdup(3), strerror(3), strlen(3), unlink(2),
and write(2).
SEE ALSO
db_archive(1), db_checkpoint(1), db_deadlock(1), db_dump(1),
db_load(1), db_recover(1), db_stat(1), db_intro(3),
db_internal(3), db_thread(3), Db(3), Dbc(3), DbEnv(3),
DbException(3), DbInfo(3), DbLock(3), DbLockTab(3), DbLog(3),
DbLsn(3), DbMpool(3), DbMpoolFile(3), Dbt(3), DbTxn(3),
DbTxnMgr(3)
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