[ovirt-devel] why we need "postZero" and discard the block device in deleteVolumes(class Image) in vdsm?

[pengyixiang]

hello, everyone!
    when Image.delete called in vdsm, we need a param "discart" in [1], then I find is just supported by block and "/sbin/blkdiscard" called in last in [2], but 
I don't know why we need to discard the block devices rather than directly use "rm -f" ? and what time we need set "discart = TRUE" ?
    And what time we should set "postZero=TRUE" ?




[1]  https://github.com/oVirt/vdsm/blob/2b98daffb2c2dec1bbbd3427d3d32861d77f01a7/lib/vdsm/API.py#L841
[2]https://github.com/oVirt/vdsm/blob/2b98daffb2c2dec1bbbd3427d3d32861d77f01a7/lib/vdsm/storage/blockSD.py#L714
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