[PATCH V2 0/1] loop: introduce LO_FLAGS_NO_DEALLOC

[Zhang Boyang <zhangboyang.id@gmail.com>]

Hi,

This patch gives userspace ability to prevent underlying file of loop
device to be sparse. Currently, if loop device is 'trimmed'
(BLKDISCARD), then underlying file will always become sparse and disk
space freed. This behaviour is good for thin provisioning but not good
for preallocated disk images. This patch introduces LO_FLAGS_NO_DEALLOC
flag, which will write zeroes to underlying file instead of punching
holes when BLKDISCARD is requested. Thus reducing file fragmentation of
preallocated disk images and improve performance.

I will also submit patches to util-linux to provide userspace support if
this patch is merged.

Changes in V1->V2:
Renamed NODEALLOC to NO_DEALLOC, to avoid confusion between NO_DEALLOC
and NODE_ALLOC. Suggested by Jens Axboe.

Best Regards,
Zhang Boyang