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Xen (hypervisor)

Definition

Xen is a general-purpose open-source hypervisor originating at the University of Cambridge. Until late 2017, every EC2 instance ran on Xen. AWS progressively moved EC2 off of Xen toward the systems/nitro / KVM-based lightweight hypervisor so that networking, storage, and isolation could be driven by dedicated hardware pipelines.

Relevance here

In the EBS retrospective, Xen is the "general-purpose hypervisor" whose IO-path choices turned out to be EBS's tax:

  • IO queues compounded. Instance block device queue → Xen ring → dom0 kernel block device queue → EBS client network queue.
  • The Cambridge-era default for Xen block-device ring-queue parameters (number of queues × queue entries) capped the EC2 host at 64 outstanding IO requests total, across all devices. This scaling accident was surfaced by patterns/loopback-isolation of each queue layer.
  • Software virtualization costs motivated the move to Nitro: first offloading VPC (networking), then EBS (storage).

Still: Xen served EC2 well enough that EBS "had served us well." The post is clear the issue wasn't Xen's fault — it was about moving out of a general-purpose design for an increasingly specialized workload.

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