[Libguestfs] [PATCH v4 4/4] file: Zero for block devices on old kernels

[Eric Blake]

On 08/19/2018 11:56 AM, Nir Soffer wrote:
> fallocate(FALLOC_FL_ZERO_RANGE) is supported for block devices with
> modern kernel, but when it is not, we fall back to manual zeroing.
> 
> For block device, try also to use ioctl(BLKZEROOUT) if offset and count
> are aligned to block device sector size.
> 

> @@ -119,9 +126,12 @@ file_config_complete (void)
>   /* The per-connection handle. */
>   struct handle {
>     int fd;
> +  bool is_block_device;
> +  int sector_size;
>     bool can_punch_hole;
>     bool can_zero_range;
>     bool can_fallocate;
> +  bool can_zeroout;

I generally try to group members by size, to avoid holes that push 
struct sizes beyond a cache line size.  Thankfully, it looks like this 
struct is now 16 bytes on both 32- and 64-bit architectures, whether or 
not you sink 'is_block_device' so that all 5 bools are contiguous.  If 
we were worried about it, we could use ':1' for the bools to make the 
struct occupy 12 bytes instead, but I doubt this code is in the hotspot 
for it to make a difference. So I'm not changing it.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org