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docs: Add warning about using a cleared image with VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES_ZEROES

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The migration parameter causes zero detection to be enabled and zero
blocks are *not* transferred to the destination. This means that users
must provide pre-cleared images that read all zero, otherwise the
non-zero blocks on destination which reside in places where the source
has zero blocks would be kept intact corrupting the image.

As not transferring and overwriting the zero blocks is what the feature
is supposed to do the users need to provide the proper environment.

Document the requirement, both in API and in the virsh man page for the
'--migrate-disks-detect-zeroes' option.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
This commit is contained in:
Peter Krempa 2024-10-10 08:42:20 +02:00
parent 04d6a0ec5d
commit 332ee4931a
2 changed files with 7 additions and 2 deletions
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4
docs/manpages/virsh.rst
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@ -3425,7 +3425,9 @@ transfer via the comma separated ``disk-list`` argument.
The *--migrate-disks-detect-zeroes* option which takes a comma separated list of 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. disk target names enables zeroed block detection for the listed migrated disks.
These blocks are not transferred or allocated on destination, effectively These blocks are not transferred or allocated on destination, effectively
sparsifying the disk at the cost of CPU overhead. sparsifying the disk at the cost of CPU overhead. Users must ensure that any
pre-created storage source is cleared and thus reads all-zeroes before using
this option as otherwise the destination image may become corrupted.
With *--copy-storage-synchronous-writes* flag used the disk data migration will With *--copy-storage-synchronous-writes* flag used the disk data migration will
synchronously handle guest disk writes to both the original source and the 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 destination to ensure that the disk migration converges at the price of possibly

5
include/libvirt/libvirt-domain.h
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@ -1245,7 +1245,10 @@ typedef enum {
* *
* virDomainMigrate* params multiple field: The multiple values that list * virDomainMigrate* params multiple field: The multiple values that list
* the block devices for which zero detection (to avoid transferring zero blocks) * 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 * is to be enabled. Users must ensure that any pre-created storage source on
* the destination will be cleared and thus read all-zeroes before using this
* feature, otherwise the destination image may become corrupted.
* This may increase CPU overhead of the migration. At the
* moment this is only supported by the QEMU driver but not for the tunnelled * moment this is only supported by the QEMU driver but not for the tunnelled
* migration. * migration.
* *