Leverage O_DIRECT

[TimJDFletcher]

Would it be possible to add an option to to open devices for reading and writing with O_DIRECT? This would prevent the large reads and writes to devices that bdsync does from polluting the buffer cache.

You can use old method of mapping a raw device to work round this but it's been deprecated since Linux 2.6.3

[rolffokkens]

Seems doable, I'll have a look into it.

[error10]

O_DIRECT doesn't work with ZFS yet. So this would still have to fall back when unavailable.

[shodanshok]

Rather than using O_DIRECT, give a look at this pull request. I am using POSIX_FADV_DONTNEED to greatly decrease cache pollution.

[rolffokkens]

Implemented shodanshok's POSIX_FADV_DONTNEED, which reduces cache pollution substantially.