On Mon, May 18, 2009 at 7:56 PM, Quanah Gibson-Mount quanah@zimbra.comwrote:

--On Monday, May 18, 2009 7:55 AM -0700 Bill MacAllister whm@stanford.edu wrote:

However, on BDB 4.7 it seems the default behavior on Linux is also to

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do synchronous flushes of the cache. As such, one approach to getting consistent performance is to configure the backend to use shared memory for the BDB cache instead of mmap'd files. That way incidental page updates don't sync to anything, and the BDB library has full control over when pages get flushed back to disk.

We can confirm that setting shm_key on 4.7 dramatically affects the load time. In our initial tests of 4.7 we just pulled our old 4.2 BDB parameters forward. We never saw the slapadd of a 4.6 gbyte database complete. We killed it after 4 hours. After changing the shm_key setting the load time dropped to the more normal 30 minutes.

To expand on this slightly. BDB has no difference in perf for small databases between disk and shm. It's only once your database is past some 6GB in size that shm vs disk starts to make a difference on Linux.

Could someone elaborate on the setting (shared memory instead of mmap) that is best suited for each major OS? Like Linux, FreeBSD, Solaris?

--Quanah

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Quanah Gibson-Mount Principal Software Engineer Zimbra, Inc

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