Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations

[Christoph Hellwig]

On Wed, Mar 14, 2012 at 01:01:35PM +0100, Kevin Wolf wrote:
> Paolo mentioned a use case as a fast way for guests to write zeros, but
> is it really faster than a normal write when we have to emulate it by a
> bdrv_write with a temporary buffer of zeros? On the other hand we have
> the cases where discard really means "I don't care about the data any
> more" and emulating it by writing zeros is just a waste of resources there.

On raw a real discard also will destroy the preallocation, which might
absolutely kill your performance on filesystems.

XFS provides a XFS_IOC_ZERO_RANGE ioctl (same calloing convention as
XFS_IOC_RESVSP), which would provide a good backend for WRITE SAME emulation
in this context.