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I am running a lot of KVM virtual machines, which open the hard disk image with the O_DIRECT flag.

I understand, according to the open() man page, that using O_DIRECT will try to bypass the Linux page cache when writing to a file, but does that apply to block devices as well? (for instance when the hard disk image is a LVM volume, or a ZFS zvol)

NB: the use case of O_DIRECT here, is that the guest is already maintaining its own page cache, so it makes no sense to duplicate that on the host.

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And it seems the answer is yes: opening a block device with O_DIRECT will bypass the page cache as well. I found the answer in this 2009 blog post about the Linux Page Cache .

It’s possible to bypass the page cache using O_DIRECT in Linux or NO_BUFFERING in Windows, something database software often does.

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  • please update the answer with a valid source. the embedded link is broken
    – aspen100
    Sep 15, 2021 at 17:46
  • I found a correct, better link.
    – Manu
    Sep 19, 2021 at 14:16

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