>> >> When I instrumented code in various places to try to track down why there >> was such a huge difference when fragsize != blocksize I found that the >> killer was repeated read-modify-write cycles, especially on filesystem >> metadata. Creating a file and writing a few bytes to it could result in >> dozens of blocks read then written, and some of the blocks got re-read >> several times in the process. It was always a mystery to me why the same >> sectors would get read over and over again (isn't that what buffer and >> filesystem caches are for?) But I know for certain the physical reads were >> happening because the instrumentation for that was in a custom raid driver >> of our own. > > Could you tell me where is your custom raid driver? I mean, is it part of > the operating system or inside the disk controller? >
It was custom hardware and software for an embedded system. We had a motherboard with 18 adaptec AIC 78xx chips on it, and our own software raid layer (modified freebsd kernel) that did special-purpose stuff (striping and error recovery using custom parity-generating hardware also on the mobo). I mentioned the raid driver only as a way of indicating that I was sure I was instrumenting real physical IO at the very lowest talk-to-the-drive layer, and I was seeing read-modify-write cycles at that layer that indicated the same sectors were being read over and over again (filesystem metadata sectors) during the higher-level operation of create-file, write 1 byte to file, close file. -- Ian To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-hackers" in the body of the message
