The idea of migration with VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES populated is to sparsify the image. The QEMU NBD client as it was configured in commit 621f879adf98e2c93ac5c8c869733a57f06cd9aa would signal to the destination to do thick allocation of holes which would result in a non-sparse image for any backend except a qcow2 image which I used to test it. Switch to VIR_DOMAIN_DISK_DETECT_ZEROES_UNMAP and VIR_DOMAIN_DISK_DISCARD_UNMAP which tells the NBD client (and that in turn the NBD server) to preserve the sparse blocks it detected from the image. Fixes: 621f879adf98e2c93ac5c8c869733a57f06cd9aa Signed-off-by: Peter Krempa <pkrempa(a)redhat.com&gt; --- src/qemu/qemu_migration.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c index 179e9d7c83..832b2946e0 100644 --- a/src/qemu/qemu_migration.c +++ b/src/qemu/qemu_migration.c @@ -1092,8 +1092,14 @@ qemuMigrationSrcNBDStorageCopyBlockdevPrepareSource(virDomainDiskDef *disk, copysrc->protocol = VIR_STORAGE_NET_PROTOCOL_NBD; copysrc->format = VIR_STORAGE_FILE_RAW; - if (detect_zeroes) - copysrc->detect_zeroes = VIR_DOMAIN_DISK_DETECT_ZEROES_ON; + if (detect_zeroes) { + /* We need to use both VIR_DOMAIN_DISK_DETECT_ZEROES_UNMAP and + * VIR_DOMAIN_DISK_DISCARD_UNMAP as the qemu NBD client otherwise singals + * to the server to fully allocate the zero blocks + */ + copysrc->detect_zeroes = VIR_DOMAIN_DISK_DETECT_ZEROES_UNMAP; + copysrc->discard = VIR_DOMAIN_DISK_DISCARD_UNMAP; + } copysrc->backingStore = virStorageSourceNew();