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storage: Use offset on (cold) merge instead of measure required size. #418

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storage: Use offset on (cold) merge instead of measure required size. #418

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16 changes: 14 additions & 2 deletions lib/vdsm/storage/merge.py
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Original file line number Diff line number Diff line change
Expand Up @@ -198,8 +198,17 @@ def _extend_base_allocation(base_vol, top_vol):
if not (base_vol.is_block() and base_vol.getFormat() == sc.COW_FORMAT):
return

# Get the latest offset
log.debug("Checking top=%r", top_vol.volUUID)
top_check = qemuimg.check(top_vol.getVolumePath(), qemuimg.FORMAT.QCOW2)
log.debug("Checking base=%r", base_vol.volUUID)
base_check = qemuimg.check(base_vol.getVolumePath(), qemuimg.FORMAT.QCOW2)

log.debug("Offset result: top: %s - base %s",
top_check["offset"], base_check["offset"])

# Measure the subchain from top to base. This gives us the required
# allocation for merging top into base.
# bitmap space
log.debug("Measuring sub chain top=%r base=%r",
top_vol.volUUID, base_vol.volUUID)
measure = qemuimg.measure(
Expand All @@ -213,7 +222,10 @@ def _extend_base_allocation(base_vol, top_vol):
# When merging we always copy the bitmaps from the top to base. Measure
# gives us the size of the bitmaps in top *and* base, so this may allocate
# more than needed, but bitmaps are small so it should be good enough.
required_size = measure["required"] + measure.get("bitmaps", 0)
# But but the data itself we sum the offset of both images to make sure we
# allocate enough space. This as measure does not give us a reliable size.
required_size = top_check["offset"] + base_check["offset"] + \
measure.get("bitmaps", 0)

# If the top volume is leaf, the base volume will become leaf after the
# merge, so it needs more space.
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