nsslapd-db-page-size

- size of database pages in bytes

Description

Specifies the size of the pages used to hold items in the database, in bytes. Valid page sizes are 512, 1024, 2048, 4096, 8192, 16384, 32768, and 65536 bytes. If the page size is not explicitly set, Directory Server defaults to a page size of 8192 bytes. Ideally, page size is set such that entries fit in database pages without wasting space. Whether it is possible to achieve an ideal database page size depends on your entries, and on whether they are subject to change in size.

For ZFS file systems, you may find performance is best when the file system block size is set to equal the database page size. For example, try setting both to 32K or 64K.

To change the value of nsslapd-db-page-size, perform the following steps.

1. Shut down the Directory Server instance using the dsadm command.

2. Back up the Directory Server databases to LDIF using the dsadm export command.

3. Change the value of the attribute in dse.ldif.

4. Restore the Directory Server databases from LDIF using the dsadm import command.

5. Restart the Directory Server instance using the dsadm command.

Changing this default value can have significant performance impact. If the page size is too small, it results in extensive page splitting and copying, whereas if the page size is too large, it can waste disk space.

You can find entries in the overflowed pages using the following command:

install-path/lib/64/ns-slapd dbtest -D instance-path -n database

The maximum size of an entry that can fit in a normal page is db-page-size/4 — 24 (24 is the per page binary tree internal structure).

For example, in a database of 40 million entries, if the database page size is 8K, and an entry size is greater than 2024 bytes then it results in an overflowed page. If all entries in the database are larger than 2024 bytes, the 40 million overflowed pages are obtained. A search operation has to perform increased number of database input and output operations, that is, the initial block of the overflowed page and the total number of continuation blocks in the overflowed page, which in turn affects the performance.

If there are no or very few overflow pages in your database, then do not change the page size. Increasing the page size may result in more lock contention and increased input and output operations. The impact depends on the file system and its page size. If possible, matching database page size to the file system page size is the best practice.

The following list describes the characteristics of this configuration attribute.

Entry DN

cn=config,cn=ldbm database,cn=plugins,cn=config

Valid Range

512 bytes to 64 KB

Default Value

8192 (bytes)

Syntax

Integer

Example

nsslapd-db-page-size: 8192

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

dse.ldif(4)