The key insight is that refcounts are not required to access data in the image. This is the motivation behind the lazy refcount optimization. Refcount metadata is not needed for guest->file block address translationĪnd therefore does not need to be on-disk at the time of write completion (hence we cannot take advantage of caching updates in RAM). Reducing metadata I/O is important for cache=writethrough and cache=directsyncīecause these modes guarantee that data is on disk after each write (in other words we need an fsck-like scan on startup) In the case of crash or power failure the image will be left in a dirty state Postpones refcount metadata updates and instead marks the image dirty. Lazy refcounts is a performance optimization for qcow2 that drive file=hd.qcow2,cache-clean-interval=900 # removes all unused cache entries every 15 minutes: The second level structures are called refcount blocks (one cluster in size) The data is stored in a two-level structure (for cluster allocation and internal snapshots) The qcow2 format mantains a reference count for each cluster. If only "cache-size" is specified then QEMU will assign as much memory as possible to the L2 cache before increasing the refcount cache size. drive file=hd.qcow2,refcount-cache-size=524288 Refcount-cache-size: maximum size of the refcount block cacheĬache-size: l2-cache-size + refcount-cache-size L2-cache-size: maximum size of the L2 table cache # Both caches must have a size that is a multiple of the cluster size With the default values for cluster_size (64KB), l2_cache_size (1Mb) QEMU keeps a cache of L2 tables in memoryĭisk_size = l2_cache_size * cluster_size / 8 QEMU needs to read its corresponding L2 table to find out where that data is located (each I/O operation) There can be many L2 tables (depending on how much space has been allocated in the image) The qcow2 image contains a set of tables organized in a two-level structure. In order to map the virtual disk as seen by the guest to the qcow2 image in the host, The virtual disk seen by the guest is also divided into guest clusters size. Qemu-img convert -O qcow2 -p -l 1514889149 win7.qcow2 win7-ii.qcow2Ī qcow2 file is organized in units of constant size called clusters. # backing_file: image will record only the differences from backing_file Qemu-img create -f qcow2 -o backing_file=rootdev.qcow2 rootdev-live.qcow2 Qemu-nbd -c /dev/nbd0 -n -aio=native /lxc/vps/ rootdev-live.qcow2 Qemu-img convert -O qcow2 /path/to/directory /lxc/vps/rootdev.qcow2 Kvm-ok 2Mbytes), preallocation(off|metadata) Processor specific module: kvm-intel.ko or kvm-amd.ko. I think it is pretty cool and a testament to the power of VirtualBox that it is able to not only boot a Linux Live CD within the Virtual Machine Harddrive, but also mount and use a Windows hard drive, inside of the virtual machine.Kvm > xen > vbox # 使用 paravirtualization device 時Īptitude install qemu-kvm libvirt-bin // 基本上 kvm 與 qemu 是密不可分的 !! It can also be used to edit the registry and even upgrade a regular account to an administrator account. Included among these tools is chntpw, which is an incredibly useful tool for resetting a forgotten windows password. System Rescue CD includes a lot of extremely useful tools, such as cfdisk, gparted, a number of data recovery tools, as well as many other tools and editors. I couldn’t tell you how many times it has saved me, both when working with Linux Systems and Windows Systems. If you haven’t ever used it, SystemRescue CD is a lightweight, yet very powerful, Linux Live CD that is based off of Knoppix Linux. Regardless, VirtualBox is pretty impressive, as it was not only able to boot the SystemRescue CD, but also I was able to use chntpw to edit the password, upgrade user privileges, and edit the registry of a Windows XP Virtual Machine. I continue to be impressed with just how powerful VirtualBox is, although to be fair I am not sure if libvrt would also do this, which I will have to test.
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