On Mon, 5 Apr 2010, Peter Schuller wrote:
It may be FreeBSD specific, but note that I a not talking about the amount of memory dedicated to the ARC and how it balances with free memory on the system. I am talking about eviction policy. I could be wrong but I didn't think ZFS port made significant changes there.
The ARC is designed to use as much memory as is available up to a limit. If the kernel allocator needs memory and there is none available, then the allocator requests memory back from the zfs ARC. Note that some systems have multiple memory allocators. For example, there may be a memory allocator for the network stack, and/or for a filesystem.
The FreeBSD kernel is not the same as Solaris. While Solaris uses a common allocator between most of the kernel and zfs, FreeBSD may use different allocators, which are not able to share memory. The space available for zfs might be pre-allocated. I assume that you have already read the FreeBSD ZFS tuning guide (http://wiki.freebsd.org/ZFSTuningGuide) and the ZFS filesystem section in the handbook (http://www.freebsd.org/doc/handbook/filesystems-zfs.html) and made sure that your system is tuned appropriately.
Why would this *not* occurr on Solaris? It seems to me that it would imply the ARC was broken on Solaris, since it is not *supposed* to be a pure LRU by design. Again, there may very well be a FreeBSD specific issue here that is altering the behavior, and maybe the extremity of it that I am reporting is not supposed to be happening, but I believe the issue is more involved than what you're implying in your response.
There have been a lot of eyeballs looking at how zfs does its caching, and a ton of benchmarks (mostly focusing on server thoughput) to verify the design. While there can certainly be zfs shortcomings (I have found several) these are few and far between.
Bob -- Bob Friesenhahn bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
