On Tue, Dec 29, 2015 at 03:36:07PM +0800, 浩樊啊 wrote: > qemu use paio or libaio finish I/O reqeusts,does that mean that when a sync > request comes from a vm becomes async request in the host?
Yes, although storage controllers (virtio-blk, SCSI, SATA, and even IDE in DMA mode) are asynchronous at the hardware interface level. That means the guest OS submits an I/O request to the device and the guest CPU continues running until the I/O completion interrupt occurs. The exceptions are old hardware interfaces like some of the SD Card controllers or maybe floppy where a single hardware register read/write actually involves synchronous I/O (the guest cannot execute CPU instructions while the I/O request is happening). Stefan
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