KVM storage performance and cache settings on Red Hat Enterprise Linux 6.2

Written by Gionatan Danti on . Posted in Virtualization

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Debian base system installation time – EXT3 and no guest-side write barriers

Now we can see how the various caching policies affect a Debian base system installation time. Will a real-world scenario redeem the historically safer write-through mode?

Debian install time - EXT3

In the case of a guest operating system with default-configured EXT3, it seems no: the writethrough policy remains way slower than the others. While the nocache mode shows remarkably good result, the write-back one is marginally faster thank to its use of not only guest-side, but also host-side pagecaches.

This is a perfect example of the safety vs speed trade-off described above: the guest OS does not use write barriers and so the writeback mode is significantly prone to data loss (because the synchronized guest-side writes are cached in the host-side pagecache before to be flushed to permanent storage) , while the writethrough mode, while slower, is even safer than this guest OS configuration running on the real hardware.

In this case, a good compromise between safety and performance is the nocache mode: it provide a safety level comparable to that achieved by this guest OS instance running on real hardware, as synchronized guest-side writes are cached in disk's internal cache only.

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