qcow2: Optimize write zero of unaligned tail cluster

We've already improved discards to operate efficiently on the tail
of an unaligned qcow2 image; it's time to make a similar improvement
to write zeroes.  The special case is only valid at the tail
cluster of a file, where we must recognize that any sectors beyond
the image end would implicitly read as zero, and therefore should
not penalize our logic for widening a partial cluster into writing
the whole cluster as zero.

However, note that for now, the special case of end-of-file is only
recognized if there is no backing file, or if the backing file has
the same length; that's because when the backing file is shorter
than the active layer, we don't have code in place to recognize
that reads of a sector unallocated at the top and beyond the backing
end-of-file are implicitly zero.  It's not much of a real loss,
because most people don't use images that aren't cluster-aligned,
or where the active layer is a different size than the backing
layer (especially where the difference falls within a single cluster).

Update test 154 to cover the new scenarios, using two images of
intentionally differing length.

While at it, fix the test to gracefully skip when run as
./check -qcow2 -o compat=0.10 154
since the older format lacks zero clusters already required earlier
in the test.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170507000552.20847-11-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 128 insertions, 0 deletions

diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
index d3b68e7729..d8485eeff2 100644
--- a/tests/qemu-iotests/154.out
+++ b/tests/qemu-iotests/154.out
@@ -282,4 +282,132 @@ read 1024/1024 bytes at offset 76800
 { "start": 69632, "length": 4096, "depth": 0, "zero": true, "data": false},
 { "start": 73728, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 77824, "length": 134139904, "depth": 1, "zero": true, "data": false}]
+
+== unaligned image tail cluster, no allocation needed ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
+wrote 512/512 bytes at offset 134219264
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
+wrote 1024/1024 bytes at offset 134218240
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
+wrote 2048/2048 bytes at offset 134217728
+2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134219264
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 1024/1024 bytes at offset 134218240
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 2048/2048 bytes at offset 134217728
+2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134219264
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 1024/1024 bytes at offset 134218240
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 2048/2048 bytes at offset 134217728
+2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+2048/2048 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134218752
+wrote 1024/1024 bytes at offset 134217728
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 512/512 bytes at offset 134218240
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134218240
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+1024/1024 bytes allocated at offset 128 MiB
+[{ "start": 0, "length": 134217728, "depth": 0, "zero": true, "data": false},
+{ "start": 134217728, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+wrote 1024/1024 bytes at offset 134217728
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+1024/1024 bytes allocated at offset 128 MiB
+read 1024/1024 bytes at offset 134217728
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+[{ "start": 0, "length": 134217728, "depth": 0, "zero": true, "data": false},
+{ "start": 134217728, "length": 1024, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
+
+== unaligned image tail cluster, allocation required ==
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 512/512 bytes at offset 134218752
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1536/1536 bytes at offset 134218240
+1.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+wrote 512/512 bytes at offset 134218240
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134218240
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1024/1024 bytes at offset 134218752
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 512/512 bytes at offset 134219264
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134217728
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 512/512 bytes at offset 134218240
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 1024/1024 bytes at offset 134218752
+1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done