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I was working on a VirtualBox (4.2.4) VM (Debian Linux 64 bit, using ext3 filesystem, host drive is NTFS) and had a BSOD on the host system while the guest was operating normally.

I had changed a source file that lives solely on the VM (no shared folders) built the application, observed that the change had occurred, and then the host crashed.

Upon reboot, the most recent change to the file was no longer present (nor were the build products). I'm wondering where those changes (which the VM thought it was reading from it's disk) were being stored if not on the host's hard drive.

I ask this because if the read is coming from memory it impacts the verisimilitude of the development environment compared to the production servers. Naturally it's not perfect, but I've had trouble finding any information related to this topic.

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VirtualBox doesn't write changes to the virtual disk instantly - those are cached instead to avoid unnecessary disk writes and thus improve the performance. Your case is mentioned here:

The caching indirectly also affects other aspects such as failure behavior, i.e. whether the virtual disk contains all data written before a host OS crash. Consult your host OS documentation for details on this.

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