On Wed, 2005-05-11 at 11:48 +0100, Chris Croome wrote:
Hi A word of caution regarding the 3ware cards -- this bug hasn't yet been solved:
- Extremely high iowait with 3Ware array and moderate disk activity https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=121434
Chris
It's not a "bug," it's the design of the 3Ware 8506 cards. Kernels change defaults regularly, which means people need to be explicitly setting the most optimal kernel parameters in their /etc/rc.d/rc.local or similar once they find them.
3Ware's site has all sorts of recommendations.
The 3Ware cards will queue up more I/O operations and off-load so much from the kernel -- far more than any other card I've seen. The problem is when implementing RAID-5 on a 7000/8000 series, the measly 2-4MB of SRAM (static RAM) overflows on even a moderate number of writes. The I/O then stalls. The key is to tweak the I/O parameters of the kernel to handle this limitation of the 3Ware 7000/8000 series.
The 3Ware 7000/8000 series cards are "storage switches" and _not_ "buffering controllers."
It's like if your NFS/Samba server could only store files in its CPU cache and didn't have main memory, before sending information to disk. SMB operations would quickly stall and have to wait on the disk writes. The majority of other controllers have the opposite problem. They are like having a CPU with no cache, just a lot of memory. I know this is an over-simplification, but that's the best analogy I can come up with.
The 3Ware 9000 series adds a good amount of DRAM for more buffering operations, such as RAID-5 writes. But they are new, and the drivers are still maturing.