Thanks to good advice from many people, here are my findings and conclusions. (1) Splitting the RAID works. I have now implemented this technique on the production system and am making a backup right now. (2) NBD is cool, works well on Debian, and is very convenient. A couple experiments suggest it may be slower compared to netcat for blasting data across the network. By slower, I mean less throughput to the point where the network can become a bottleneck. I don't have conclusive data yet, so take with a large grain of salt. I am using netcat for now. (3) End-to-end throughput is not quite as high as I'd hoped. At this point it appears the limiting factor is the speed (throughput) of the destination disks. During earlier testing, I had been dumping bits to /dev/null on the destination machine instead of the actual destination partition. No worries, this can be addressed. (4) I'll play with fancier options like "write-mostly" when Debian releases a 2.6.13 kernel, and when I'm convinced that I'm not going accidentally introduce slower disks into the RAID and bog the entire system down for writes. >sendfile() bypasses the copy to user buffer, which in turn will bypass >copy to system buffers, which eliminates contention for buffer space. Use >vmstat to check, if you have a lot of system time and lots of space in >buffers of various kinds, there's a good possibility that the problem is >there. I use the -d option in dd_rescue, which invokes O_DIRECT and therefore doesn't trash Linux's disk buffer. Unfortunately because of the very random access patterns of the web server, cache misses are extremely common anyway. Cheers, Jeff - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
