Can i create DB in shared mem, rather than backed by a file

Question

Hi
I'm using db 4.2.6 version and trying to create a DB in shared mem rather than a file. How do I achieve that.
While I look at documentation here:
http://www.oracle.com/technology/documentation/berkeley-db/db/programmer_reference/env_region.html
==========
If the DB_SYSTEM_MEM flag is specified to DB->open(), shared regions are created in system memory rather than files. This is an alternative mechanism for sharing the Berkeley DB environment among multiple processes and multiple threads within processes.
===========
But DB->open() rejects DB_SYSTEM_MEM flat out. db->open() doesn't even talk about flag:
http://www.oracle.com/technology/documentation/berkeley-db/db/api_reference/C/dbopen.html
I could specify DB_SYSTEM_MEM for DBENV.
Here is my code. Also DBENV->open() won't let me specify DB_PRIVATE.
flags = DB_INIT_MPOOL | DB_CREATE | DB_SYSTEM_MEM | DB_INIT_TXN | DB_RECOVER | DB_INIT_LOG |
            DB_LOG_AUTOREMOVE;
dbenv->open(dbenv, dbhome, flags, 0);
flags = DB_CREATE | DB_EXCL| DB_AUTO_COMMIT;
dbp->open(dbp, NULL, dbfile, NULL, DB_BTREE, flags, 0);If I create db as above, where is it getting created? I don't see it here:
# ./bdbperf -t -i 10000 -k 32 -v 128;ls -la dbtest
create log/transaction
creating in system mem
inserted 10000 records. errs 0
logs removed
inserting 10000 records took 0s.290ms (29 micros/op)
done. 0s.290ms
total 200
drwxr-xr-x 2 root root 80 Jul 17 03:04 .
drwxrwxrwx 7 root root 1180 Jul 17 03:04 ..
-rw-r----- 1 root root 8 Jul 17 03:04 __db.001
-rw-r----- 1 root root 190424 Jul 17 03:04 log.0000000011
However, if I specify a filename arg in dbp->open(), I do see a file with that name in my env dir 'dbtest'. db_stat on such file seems to be good.
# ./lt-db_stat -d dbfile -h dbtest
Sat Jul 17 02:38:01 2010 Local time
53162 Btree magic number
9 Btree version number
Little-endian Byte order
Flags
2 Minimum keys per-page
1024 Underlying database page size
239 Overflow key/data size
4 Number of levels in the tree
10000 Number of unique keys in the tree
10000 Number of data items in the tree
171 Number of tree internal pages
79122 Number of bytes free in tree internal pages (54% ff)
1822 Number of tree leaf pages
738356 Number of bytes free in tree leaf pages (60% ff)
0 Number of tree duplicate pages
0 Number of bytes free in tree duplicate pages (0% ff)
0 Number of tree overflow pages
0 Number of bytes free in tree overflow pages (0% ff)
0 Number of empty pages
0 Number of pages on the free list
So:
1. how do I create DB in shared-mem area and
2. how do I get stats for such DB?
Appeciate any help.

Answer 1

Hi,
user3143985 wrote:
I'm using db 4.2.6 version and trying to create a DB in shared mem rather than a file. How do I achieve that.There must be a typo, as there is no 4.2.6 release.
user3143985 wrote:
While I look at documentation here:
http://www.oracle.com/technology/documentation/berkeley-db/db/programmer_reference/env_region.html
When reading the documentation, I would indicate to read the documentation from the release package you downloaded. The online documentation is accurate for the latest BDB release.
user3143985 wrote:
==========
If the DB_SYSTEM_MEM flag is specified to DB->open(), shared regions are created in system memory rather than files. This is an alternative mechanism for sharing the Berkeley DB environment among multiple processes and multiple threads within processes.
===========
But DB->open() rejects DB_SYSTEM_MEM flat out. db->open() doesn't even talk about flag:
http://www.oracle.com/technology/documentation/berkeley-db/db/api_reference/C/dbopen.html
That's a documentation error, it should refer to the DB_ENV->open()method. Thanks for pointing it out! As you can see, the DB_SYSTEM_MEM is linked to the DB_ENV->open() method: http://www.oracle.com/technology/documentation/berkeley-db/db/api_reference/C/envopen.html#envopen_DB_SYSTEM_MEM
user3143985 wrote:
1. how do I create DB in shared-mem area and This page should clarify this question and how a memory-only configuration looks like: http://www.oracle.com/technology/documentation/berkeley-db/db/programmer_reference/program_ram.html
user3143985 wrote:
2. how do I get stats for such DB?If you want to allocate region memory from the heap instead of from memory backed by the filesystem or system shared memory, you'll have to specify DB_PRIVATE ( http://www.oracle.com/technology/documentation/berkeley-db/db/api_reference/C/envopen.html#envopen_DB_PRIVATE ). DB_PRIVATE should not be specified if more than a single process is accessing the environment because it is likely to cause database corruption and unpredictable behavior. For example, if both a server application and Berkeley DB utilities (for example, db_archive, db_checkpoint or db_stat) are expected to access the environment, the DB_PRIVATE flag should not be specified.
Bogdan Coman