Bug 1130506 - RFE: virt-sparsify in copying mode should support qemu detect-zeroes
Summary: RFE: virt-sparsify in copying mode should support qemu detect-zeroes
Status: NEW
Alias: None
Product: Virtualization Tools
Classification: Community
Component: libguestfs
Version: unspecified
Hardware: Unspecified
OS: Unspecified
Target Milestone: ---
Assignee: Richard W.M. Jones
QA Contact:
Depends On:
TreeView+ depends on / blocked
Reported: 2014-08-15 12:53 UTC by Richard W.M. Jones
Modified: 2018-07-18 14:29 UTC (History)
2 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
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Description Richard W.M. Jones 2014-08-15 12:53:57 UTC
Description of problem:

Since May 2014, qemu has supported a block device "detect-zeroes"
option, which turns writes of zeroes into either an efficient
zero-write operation or an unmap.

I had previously tried such an approach in qemu (see:
https://lists.gnu.org/archive/html/qemu-devel/2012-07/msg03748.html )
and found that it makes virt-sparsify in normal copying mode
far more space efficient.  The reason is that virt-sparsify writes
a big file of just zeroes to an overlay file, and if detect_zeroes
is enabled then the overlay file doesn't actually need to grow much
to handle this (qcow2 can optimize).

So we should enable this feature in virt-sparsify and
the underlying libguestfs API.

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