On Mon, Aug 11, 2014 at 02:06:54PM -0500, Andrew Martin wrote: > Hello, > > I am running several virtualization servers with QEMU 1.4.x and > libvirt 1.0.2 on Ubuntu 12.04 and am working on optimizing the cache= > and aio= options for the virtual machines. These VM images are mostly > qcow2, and are served both from a local ext4 filesystem (with > data=ordered,barrier) and from an NFS mountpoint (with sync). The > local filesystem sits on top of an md software RAID of SATA HDDs. > > I have read some conflicting information about which cache option is > used by default. This documentation states that cache=writethrough is > the default: > http://www-01.ibm.com/support/knowledgecenter/linuxonibm/liaat/liaatbpkvmguestcache.htm?lang=en The above sounds incorrect (refer below). > However this SuSE documentation claims that QEMU 1.2.x and newer > allows the driver to select which cache mode, and it often defaults to > cache=writeback: > https://www.suse.com/documentation/sles11/book_kvm/data/sect1_1_chapter_book_kvm.html The above seems correct. Looking at `qemu-img` source[1], 'cache=writeback' seems to be the default. That's also corroborated by this[2] (Rich's blog, and libguestfs/virt-tools lead developer). [1] http://git.qemu.org/?p=qemu.git;a=blob;f=qemu-img.c;h=d4518e724f848a6ff8ffaf61656d080de5a08f03;hb=HEAD#l55 [2] http://rwmj.wordpress.com/2013/09/02/new-in-libguestfs-allow-cache-mode-to-be-selected/ > Which is correct? "cache=writeback" > How is the cache mode set by default (if cache= is not > specified)? It's compiled into the binary. > My second question is can cache=none be used safely on a local ext4 > filesystem with no BBU? Since ext4 uses barriers, would writing to > these qcow2 image files be safe? The kernel documentation about > barriers states that "Write barriers enforce proper on-disk ordering > of journal commits, making volatile disk write caches safe to use, at > some performance penalty". Does this apply to qcow2 VM images? FWIW, in my test environments (which I should admit - there's not a whole lot of I/O activity), I use: $ qemu-img create -f qcow2 -o preallocation=metadata test1.qcow2 8G Followed by an `fallocate`: $ fallocate -l 8589934592 test1.qcow2 Then, I used to invoke QEMU "cache=none" (setting it in libvirt's guest XML), but lately started using the default "cache=writeback" after the I learnt about the bug from Rich's blog above. -- /kashyap _______________________________________________ libvirt-users mailing list libvirt-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvirt-users
