20 lines
970 B
YAML
20 lines
970 B
YAML
---
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fixes:
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- |
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Update the way QEMU cache mode is configured for Nova guests: If the
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file system hosting the directory with Nova instances is capable of
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Linux's O_DIRECT, use ``none``; otherwise fallback to ``writeback``
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cache mode. This improves performance without compromising data
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integrity. `Bug 1818847`_.
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Context: What makes ``writethrough`` so safe against host crashes is
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that it never keeps data in a "write cache", but it calls fsync()
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after *every* write. This is also what makes it horribly slow. But
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cache mode ``none`` doesn't do this and therefore doesn't provide
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this kind of safety. The guest OS must explicitly flush the cache
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in the right places to make sure data is safe on the disk; and all
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modern OSes flush data as needed. So if cache mode ``none`` is safe
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enough for you, then ``writeback`` should be safe enough too.
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.. _Bug 1818847: https://bugs.launchpad.net/nova/+bug/1818847
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