Re: [Qemu-devel] [PATCH 4/5] blockdev: enable discard by default

[Kevin Wolf]

On Fri, Feb 08, 2013 at 02:06:14PM +0100, Paolo Bonzini wrote:
> Because discard is now a host parameter, we can always fake it as enabled
> in the guest.  This is an extension of the current choice to ignore
> "not supported" errors from the host when discard_granularity is set
> to nonzero.
> 
> The default granularity is set to the logical block size or 4k, whichever
> is largest, because cluster sizes below 4k are rarely used and 4K is a
> typical block size for files.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>

I wonder if it would make sense to default to 64k so that a qcow2 image
with default options can make use of it. On the other hand, it might
just mean that discard requests are already dropped in the guest kernel
instead of qemu, then we don't really win much.

Kevin