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<think>
Alright, let's break down this bug report to figure out the correct category. The user is experiencing blue screens when using cache='writeback' on their KVM guest running Windows 2008R2 under an Ubuntu hypervisor.
First, I notice they're using Virtio devices with specific drivers, which are known for better performance and support in KVM environments. They changed the disk controller's cache mode from the default 'writethrough' to 'writeback', which caused occasional blue screens linked to high disk IO operations. This suggests that changing this setting is directly related to the issue.
The problem occurs during heavy disk operations, like restoring large backups or when performing Windows updates, which are both I/O-intensive tasks. When idle, the system runs fine for extended periods, implying the issue only surfaces under load. Reverting the cache mode fixes the problem, pointing to an instability introduced by 'writeback' caching.
Considering the categories provided: kernel, hypervisor, KVM, etc., this seems related to how the disk I/O is handled within the virtualization setup. Since it's a QEMU/KVM issue tied to disk caching behavior during high I/O, which affects stability, the most fitting category would be 'hypervisor' as it pertains to the virtual machine management and hardware emulation aspects.
</think>
The bug report describes an issue where enabling cache='writeback' on a KVM guest causes blue screens under heavy disk IO. The problem is related to disk caching behavior within QEMU/KVM, which affects the stability of the guest operating system during high I/O operations. This falls under the hypervisor category as it pertains to how the virtual machine's hardware (disk controller) is emulated and configured.
**hypervisor**
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