Ok, thanks for taking time. I'll dig into your answers. So as i run relative.img on diskless systems with original.img on nfs, what are the best practice/tips i can use ? Is ramfs more suitable than tmpfs ? Fred. On Thu, 16 Sep 2010 11:09:49 +0200 Andre Przywara <andre.przywara@xxxxxxx> wrote: > TOURNIER Frédéric wrote: > > Hi ! > > Here's my config : > > > > Version : qemu-kvm-0.12.5, qemu-kvm-0.12.4 > > Hosts : AMD 64X2, Phenom and Core 2 duo > > OS : Slackware 64 13.0 > > Kernel : 2.6.35.4 and many previous versions > > > > I use a PXE server to boot semi-diskless (swap partitions and some local stuff) stations. > > This server also serves a read-only nfs folder, with plenty of .img on it. > > When clients connects, a relative image is created in /tmp, which is a tmpfs, so hosted in ram. > > > > And here i go on my 2G stations : > > qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 /tmp/relimg.img > > qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 /dev/shm/relimg.img > > > > I tried both. Always the same result : the ram is consumed quickly, and mass swap occurs. > Which is only natural, as tmpfs is promising to never swap. So it will > take precedence over other RAM (that's why it is limited to half of the > memory by default). As soon as the guest has (re)written more disk > sectors than your free RAM can hold, the system will start to swap out > your guest RAM (and other host applications). > So in general you should avoid putting relative disk images to tmpfs if > your host memory is limited. As a workaround you could try to further > limit the tmpfs max size (mount -t tmpfs -o size=512M none /dev/shm), > but this could lead to data loss in your guest as it possibly cannot > back the written sectors anymore. > > On a 4G system, i see kvm uses more than 1024, maybe 1200. > > And everytime a launch a program inside the vm, the amount of the host free ram (not cached) diminishes, which is weird, because it should have been reserved. > KVM uses on-demand paging like other applications. So it will not > reserve memory for your guest (unless you use hugetlbfs's -mempath): > $ kvm -cdrom ttylinux_ser.iso -nographic -m 3072M > $ top > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND > > 6015 user 20 0 3205m 128m 3020 S 2 2.2 0:04.94 kvm > > > Regards, > Andre. > > > > > So on a 2G system, swap occurs very fast and the machine slow a lot down. > > An on a total diskless system, this leads fast to a freeze. > > > > I have no problem if i use a relative image on disk : > > qemu-system-x86_64 -m 1024 -vga std -usb -usbdevice tablet -localtime -soundhw es1370 -drive file=/mnt/hd/sda/sda1/tmp/relimg.img,cache=none > > -- > Andre Przywara > AMD-Operating System Research Center (OSRC), Dresden, Germany > Tel: +49 351 448-3567-12 > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
