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Understanding QCOW2 Risks with QEMU Cache=None in Proxmox

a day ago
https://kb.blockbridge.com/technote/proxmox-qemu-cache-none-qcow2/
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  • #QCOW2
  • #DataDurability
  • #QEMU
  • QEMU's cache=none mode bypasses the host page cache but introduces complexities with QCOW2, leading to potential data corruption during crashes.
  • QCOW2 metadata remains in volatile memory until a flush is issued, unlike raw devices which write directly to storage, increasing risk of data loss.
  • Subcluster allocation in QCOW2 improves performance but amplifies risks of torn writes and data inconsistency during power failures.
  • Flushes and barriers are critical for ensuring data durability and write ordering in QCOW2, especially for applications bypassing filesystem journaling.
  • Raw storage devices (e.g., NVMe, iSCSI, Ceph) are safer for critical workloads due to their direct and predictable I/O behavior.
  • Modern filesystems (ext4, XFS, ZFS) help mitigate QCOW2 risks by frequently flushing journals, but applications bypassing these are more vulnerable.
  • QCOW2 with cache=none is not inherently unsafe but requires careful management of flushes and barriers to avoid data corruption.
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