[PATCH 2/2] docs: Note that zero detection on migration sparsifies image only when discard='unmap' is set
From: Peter Krempa <pkrempa(a)redhat.com>
The mirroring job clears the destination to ensure that the guest
visible disk contents are identical to the state on the source. The
image itself is kept sparse only when the disk 'discard' option is set
to 'unmap' (Also the disks would eventually desparsify itself anyways
with disabled discards). Note it in the docs for the user.
Signed-off-by: Peter Krempa <pkrempa(a)redhat.com>
---
docs/manpages/virsh.rst | 5 +++--
include/libvirt/libvirt-domain.h | 9 +++++----
2 files changed, 8 insertions(+), 6 deletions(-)
diff --git a/docs/manpages/virsh.rst b/docs/manpages/virsh.rst
index 9b571761b8..a2dab293ad 100644
--- a/docs/manpages/virsh.rst
+++ b/docs/manpages/virsh.rst
@@ -3641,8 +3641,9 @@ host. By default only non-shared non-readonly images are
transferred. Use
transfer via the comma separated ``disk-list`` argument.
The *--migrate-disks-detect-zeroes* option which takes a comma separated list of
disk target names enables zeroed block detection for the listed migrated disks.
-These blocks are not transferred or allocated on destination, effectively
-sparsifying the disk at the cost of CPU overhead.
+These blocks are not transferred or allocated (requires that 'discard' option
+on given disk is set to 'unmap') on destination, effectively sparsifying the
+disk at the cost of CPU overhead.
With *--copy-storage-synchronous-writes* flag used the disk data migration will
synchronously handle guest disk writes to both the original source and the
destination to ensure that the disk migration converges at the price of possibly
diff --git a/include/libvirt/libvirt-domain.h b/include/libvirt/libvirt-domain.h
index 1af5045571..3d05ffc5aa 100644
--- a/include/libvirt/libvirt-domain.h
+++ b/include/libvirt/libvirt-domain.h
@@ -1275,10 +1275,11 @@ typedef enum {
* VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES:
*
* virDomainMigrate* params multiple field: The multiple values that list
- * the block devices for which zero detection (to avoid transferring zero blocks)
- * is to be enabled. This may increase CPU overhead of the migration. At the
- * moment this is only supported by the QEMU driver but not for the tunnelled
- * migration.
+ * the block devices for which zero detection (to avoid transferring zero blocks,
+ * for storage where it can't be probed) is to be enabled. This may increase CPU
+ * overhead of the migration. Destination image will be sparse only when the
+ * disk 'discard' option is set to 'unmap'. At the moment this is only
supported
+ * by the QEMU driver but not for the tunnelled migration.
*
* Since: 10.9.0
*/
--
2.49.0