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

[Idan Shaby]

Hi,

The fields "discard" and "postZero" are sent to vdsm by the engine in order
to implement the "Discard After Delete" and "Wipe After Delete" features.

When a storage domain is marked with Discard After Delete, a disk that is
removed from that storage domain is discarded before it's removed. This
way, the disk's blocks are freed from the storage array and can be used by
other LUNs in the array.
For more information about Discard After Delete, please refer to the
feature page [1].

When a disk (on a block storage domain) is marked with Wipe After Delete,
it is wiped right before it's removed. The idea behind this is that when
the disk contains sensitive data, the administrator doesn't want it to be
read after it is removed. However, when we don't wipe a disk's blocks
before it is removed, the data can be read by a new VM user with a disk
that is created on some/all the blocks of the old disk.
For more information about Wipe After Delete, please refer to the
administrator guide [2].

If you have any further questions, please don't hesitate to ask.

Regards,
Idan

[1]
https://www.ovirt.org/develop/release-management/features/storage/discard-after-delete/
[2]
https://www.ovirt.org/documentation/admin-guide/chap-Virtual_Machine_Disks/#settings-to-wipe-virtual-disks-after-deletion

On Wed, Jan 3, 2018 at 4:33 AM, pengyixiang <yxpengi386 at 163.com> wrote:

> 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/2b98daffb2c2dec1bbbd3427d3d328
> 61d77f01a7/lib/vdsm/API.py#L841
> [2]https://github.com/oVirt/vdsm/blob/2b98daffb2c2dec1bbbd3427d3d328
> 61d77f01a7/lib/vdsm/storage/blockSD.py#L714
>
>
>
>
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