> > I have a RAID 1 whose write performance I tested by writing a 10 GB file but under which kernel? > > Looking at GKrellM I noticed the CPU usage is very jumpy, going from a that's some sort of gui monitoring tool, right? I usually use "setrealtime vmstat 1" for this kind of thing, since it's at least a layer or two closer to the true numbers. > > few to 99 percent (but usually is roughly rovers around 50 percent). > > Moreover, the transfer regularly stops for a few seconds (the CPU usage > > is then about 2 percent). The average data transfer rate was 16 MB/s, > > while the disks alone can make almost 25 MB/s. sounds a bit like a combination of poor VM (certainly the case for the VM in some kernels), and possibly /proc/sys/vm settings. > The next thing to look for is interrupt sharing. I've found a lot of I doubt this is an issue - shared interrupts can result in a few extra IOs per interrupt (as the wrong driver checks its device), but I'd be very surprised to find this affecting performance unless the device is very slow or the irq rate very high (1e5 or so). > > Is this normal behavior? Can the write performance be tuned (to be less > > "jumpy")? certainly. in 2.4 kernels, it was trivial to set bdflush to wake up every second, rather than every 5 seconds (the default). I do this on a fairly heavily loaded fileserver, since the particular load rarely sees any write-coalescing benefit past a few ms. > Interupts (and/or more likely the controllers) seem to me to be the > biggest bug/feature of a modern motherboard )-: I've seen systems work modern low-end motherboard, perhaps. > > Maybe the RAID 1 is just not suited for video capture? it's fine; the problem, if any, is your config. what's the bandwidth you need to write? what's the bandwidth of your disks (after accounting for the fact that every block is written twice)? you should have a couple seconds of buffer, at least, to "speed-match" the two rates, even if your producer (capture) is significantly slower than the bandwidth of your raid1. note also that your capture card could well be eating lots of cpu and/or perturbing the kernel's vm. regards, mark hahn. - 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
