[Libguestfs] [PATCH libnbd] copy: Preserve the host page cache when writing to local files.

[Richard W.M. Jones]

This is the equivalent patch for writing, using Linus's recommended
technique.  It's actually not as bad -- codewise -- as I thought it
would be, but there's a significant performance penalty (25%) still
which might be addressed by using more than two write windows.

Rich.