Am 27.02.2014 12:07, schrieb Kevin Wolf: > Am 27.02.2014 um 02:10 hat Fam Zheng geschrieben: >> On Wed, 02/26 16:41, Stefan Hajnoczi wrote: >>> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote: >>>> I was wondering if it would be a good idea to set the O_DIRECT mode for >>>> the source >>>> files of a qemu-img convert process if the source is a host_device? >>>> >>>> Currently the backup of a host device is polluting the page cache. >> Peter, can you give some more detailed explanation of the issue? An example >> or >> benchmark will help a lot. As I understand, one time scanning of a file >> doesn't >> promote the page cache to active list, so it probably won't hurt real hot >> cache >> at all, and will get replaced very soon. >> >> Considering readahead and page cache on metadata, I'm not sure if forcing >> O_DIRECT is a good idea. >> >>> The problem is what to do for image formats. An image file can be >>> very fragmented so the readahead might not be a win. Does this mean >>> that for image formats we should tell the kernel access will be >>> random? >>> >>> Furthermore, maybe it's best to do readahead inside QEMU so that even >>> network protocols (nbd, iscsi, etc) can get good performance. They >>> act like O_DIRECT is always on. >> Also, experience with booting a network backed guest can be greatly improved, >> because sometimes BIOS and bootloader are simple minded and load a kernel or >> initrd by sending thousands of 1 sector requests with iodepth=1, which means >> latency of network based IO hurts a lot. > I think I mentioned it a while ago, but our IDE emulation plays a role > in this as well. PIO requests are always handled sector by sector, no > matter how big the request was that we got from the BIOS. Yes, you have pointed that out before. How complicated is it to fix this?
Peter > > Kevin > >> Doing readahead in QEMU makes sense for image formats as well, because we >> know >> where the next data cluster is better than kernel. But again, we are >> replicating things from kernel.