blkdiscard/
sparse/thin-provisioned device,like ssd
fstrim--- discard unused blocks on a mounted filesystem
parted,partprobe(remount the partition table)---smartctl -t long /dev/sda
https://en.wikipedia.org/wiki/Device_mapper
http://www.infoq.com/cn/articles/analysis-of-docker-file-system-aufs-and-devicemapper
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