Ulrich Windl wrote:
>>> Brent Bice <bbice@sgi.com> schrieb am 18.09.2013 um 22:01 in NachrichtI've started testing an LDAP server here using MDB and ran across a few caveats that might be of use to others looking into using it. But first off, let me say a hearty THANKS to anyone who's contributed to it. In this first OpenLDAP server I've converted over to MDB it's *dramatically* faster and it's definitely nice to not worry about having to setup script/s to occasionally (carefully) commit/flush DB logs, etc.
One caveat that might be worth mentioning in release notessomewhere... Not all implementations of memory mapped I/O are created equal. I ran into this a long time back when I wrote a multi-threaded quicksort program for a friend to had to sort text files bigger than 10 gigs and didn't want to wait for the unix sort command. :-) The program I banged together for him used memory mapped I/O and one of the things I found was that while Solaris would let me memory map a file bigger than I had physical or virtual memory for, linux wouldn't. It appeared that
I doubt that Solaris allows you to mmap() a file to an area larger than the
virtual address space, however you can mmap() a file area larger than RAM+swap when a demand paging strategy is used. However once you start modifying the mapped pages you may run out of memory, so thing twice.
No OS can let you mmap a single region larger than the address space. But mapping a file larger than RAM is no problem, the OS will swap pages in and out as needed.
some versions of the 2.x kernels wouldn't let me memory-map a file bigger than the total *virtual* memory size, and I think MDB is running into the same limitation. On a SLES11 system, for instance with the 2.6.32.12 kernel, I can't specify a maxsize bigger than the total of my physical memory and swap space. So just something to keep in mind if
Also be aware that in SLES11 SP2 the kernel update release some weeks ago
strengthened the checks for mmap()ed areas: I had a program that started to fail when I tried to change one byte after the end of the file, while this worked with the kernel before.
Irrelevant for LMDB since we never do such a thing.
you're using MDB on the 2.x kernels - you may need a big swap area even though the memory mapped I/O routines in the kernel seem to be smart enough to avoid swapping like mad.
I'd like to object: AFAIR, MDB used mmap()ed areas in strictly read-only
fashion, so the backing store is the original file, being demand paged. When data is write()n, the system will dirty buffers in real RAM that are eventually written back to the file blocks. I see no path where dirty buffers should be swapped unless the mapping is PRIVATE.
Correct; since LMDB uses an mmap'd file it will *never* use swap space.