Setting the Mailbox Database Cache Size

Messaging Server makes frequent calls to the mailbox database. For this reason, it helps if this data is returned as quickly as possible. A portion of the mailbox database is cached to improve Message Store performance. Setting the optimal cache size can make a big difference in overall Message Store performance. You set the size of the cache with the configutil parameter store.dbcachesize.

You should use the configutil parameter store.dbtmpdir to redefine the location of the mailbox database to /tmp, that is, /tmp/mboxlist.

The mailbox database is stored in data pages. When the various daemons make calls to the database (stored, imapd, popd), the system checks to see if the desired page is stored in the cache. If it is, the data is passed to the daemon. If not, the system must write one page from the cache back to disk, and read the desired page and write it in the cache. Lowering the number of disk read/writes helps performance, so setting the cache to its optimal size is important.

If the cache is too small, the desired data will have to be retrieved from disk more frequently than necessary. If the cache is too large, dynamic memory (RAM) is wasted, and it takes longer to synchronize the disk to the cache. Of these two situations, a cache that is too small will degrade performance more than a cache that is too large.

Cache efficiency is measured by hit rate. Hit rate is the percentage of times that a database call can be handled by cache. An optimally sized cache will have a 99 percent hit rate (that is, 99 percent of the desired database pages will be returned to the daemon without having to grab pages from the disk). The goal is to set the cache so that it holds a number of pages such that the cache will be able to return at least 95 percent of the requested data. If the direct cache return is less than 95 percent, then you need to increase the cache size.

The database command db_stat can be used to measure the cache hit rate. In the following example, the configutil parameter store.dbtmpdir has redefined the location of the mailbox database to /tmp, that is, /tmp/mboxlist. The db_stat command is run against this location:

# /opt/SUNWmsgsr/lib/db_stat -m -h /tmp/mboxlist

2MB 513KB 604B  Total cache size.
1                   Number of caches.
2MB 520KB           Pool individual cache size.
0                   Requested pages mapped into the process’ address space.
55339               Requested pages found in the cache (99%).

In this case, the hit rate is 99 percent. This could be optimal or, more likely, it could be that the cache is too large. The way to test this is to lower the cache size until the hit rate moves to below 99 percent. When you hit 98 percent, you have optimized the DB cache size. Conversely, if db_stat shows a hit rate of less than 95 percent, then you should increase the cache size with the store.dbcachesize parameter. The maximum size is the total of all the *.db files in the store/mboxlist directory. The cache size should not exceed the total size of all of the .db files under the store/mboxlist directory.

As your user base changes, the hit rate can also change. Periodically check and adjust this parameter as necessary. This parameter has an upper limit of 2 GB imposed by the database.