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[Qemu-devel] [PATCH v4 00/17] qcow2: Add new overlap check functions
[Qemu-devel] [PATCH v4 00/17] qcow2: Add new overlap check functions
Mon, 4 May 2015 21:15:36 +0200
This is a continuation of previous versions of this series. v2's cover
letter was the most elaborate, which you can find here (includes
See patch 1 for an explanation of why this series exists and what it
does. Patch 1 is basically the core of this series, the rest just
employs the functions introduced there.
The new thing about v4 is that there now is a way to limit the total
combined size of all data structures used for the new overlap prevention
algorithm. Bad news: The limit is disabled by default. This is mainly
because for image creation, it is pretty hard to find a sane default
(it depends on the desired image size; but the image is not created with
that size, but rather with size 0 and then truncated; inferring the
default from the image size seems to make sense, but we cannot do this
for images that are going to be truncated).
RFC: An alternative would be to infer the limit like so:
MAX(1M, guest_image_size / 1M). It seems sane to me and I guess it
should work even for images with very, very large guest sizes. But I
think it will break if you're trying to preallocate like a 1 PB image
(which is not really a concern right now, but let's see about it in
On the other hand, the worst that happens is that the event is generated
and the overlap checks won't be performed for some parts of the image
during its creation. Doesn't sound too bad to me. What do you think,
In a later patch, we may want to change the meaning of the "constant"
overlap checking option to mean the same as "cached", which is
everything except for inactive L2 tables. This series does make
checking for overlaps with inactive L2 tables at runtime just as cheap
as everything else (constant time plus caching), but using these checks
means qemu has to read all the snapshot L1 tables when opening a qcow2
file. This does not take long, of course, but it does result in a bit of
overhead so I did not want to enable it by default.
- Patch 1:
- Use the length of bs->file instead of bs itself as a basis on how
many windows should be allocated initially
- If the fragment list would take up more memory than the bitmap
(which is conceivable, considering that every cluster in the
bitmap/bytemap uses exactly one byte, whereas every cluster may use
up to one whole entry in the fragment list (which takes up four
bytes)), do not drop the bitmap on cache eviction, but rather just
abort the generation of the fragment list
- The cache_size parameter should be divided by WINDOW_SIZE only
instead of WINDOW_SIZE + sizeof(*s->metadata_list->cached_windows);
the user is more likely to specify multiples of WINDOW_SIZE, so we
should take that into account (the sizeof(...) part is really
negligible, and also, if the user really wants to limit the memory
usage, there are now four new patches in this series to do that)
- Just drop the bitmap when the metadata list is destroyed; do not use
destroy_window_bitmap() to do so, because this will (unneccessarily)
generate a fragment list (which is going to be dropped, too, anyway)
- Patch 13: Added; adds a new QMP event to be used by patch 14
- Patch 14: Added; adds a control mechanism to limit the amount of
memory used for the overlap structures (and issues the event from
patch 13 if that limit is hit)
- Patch 15: Added; adds runtime options to qcow2 to control the cache
size and the total memory limit for the overlap checking
- Patch 16: Added; exposes these options via QMP
- Patch 17: Added; some tests for these new options
git-backport-diff against v3:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively
001/17: [FC] 'qcow2: Add new overlap check functions'
002/17:[----] [--] 'qcow2: Pull up overlap check option evaluation'
003/17:[----] [-C] 'qcow2: Create metadata list'
004/17:[----] [-C] 'qcow2/overlaps: Protect image header'
005/17:[----] [--] 'qcow2/overlaps: Protect refcount table'
006/17:[----] [-C] 'qcow2/overlaps: Protect refcount blocks'
007/17:[----] [--] 'qcow2/overlaps: Protect active L1 table'
008/17:[----] [-C] 'qcow2/overlaps: Protect active L2 tables'
009/17:[----] [-C] 'qcow2/overlaps: Protect snapshot table'
010/17:[----] [--] 'qcow2/overlaps: Protect inactive L1 tables'
011/17:[----] [-C] 'qcow2/overlaps: Protect inactive L2 tables'
012/17:[----] [--] 'qcow2: Use new metadata overlap check function'
013/17:[down] 'qcow2/overlaps: Add "memory limit reached" event'
014/17:[down] 'qcow2/overlaps: Add memory usage limit'
015/17:[down] 'qcow2: Add overlap structure memory size options'
016/17:[down] 'qapi: Expose new qcow2 overlap check options'
017/17:[down] 'iotests: Test qcow2's overlap check memory limit'
Max Reitz (17):
qcow2: Add new overlap check functions
qcow2: Pull up overlap check option evaluation
qcow2: Create metadata list
qcow2/overlaps: Protect image header
qcow2/overlaps: Protect refcount table
qcow2/overlaps: Protect refcount blocks
qcow2/overlaps: Protect active L1 table
qcow2/overlaps: Protect active L2 tables
qcow2/overlaps: Protect snapshot table
qcow2/overlaps: Protect inactive L1 tables
qcow2/overlaps: Protect inactive L2 tables
qcow2: Use new metadata overlap check function
qcow2/overlaps: Add "memory limit reached" event
qcow2/overlaps: Add memory usage limit
qcow2: Add overlap structure memory size options
qapi: Expose new qcow2 overlap check options
iotests: Test qcow2's overlap check memory limit
block/Makefile.objs | 3 +-
block/qcow2-cluster.c | 13 ++
block/qcow2-overlap.c | 572 +++++++++++++++++++++++++++++++++++++++++++++
block/qcow2-refcount.c | 202 +++++++---------
block/qcow2-snapshot.c | 105 ++++++++-
block/qcow2.c | 162 +++++++++----
block/qcow2.h | 15 ++
docs/qmp/qmp-events.txt | 28 +++
qapi/block-core.json | 37 +++
qapi/event.json | 27 +++
tests/qemu-iotests/060 | 222 ++++++++++++++++++
tests/qemu-iotests/060.out | 47 ++++
tests/qemu-iotests/group | 2 +-
13 files changed, 1262 insertions(+), 173 deletions(-)
create mode 100644 block/qcow2-overlap.c
- [Qemu-devel] [PATCH v4 00/17] qcow2: Add new overlap check functions,
Max Reitz <=
- [Qemu-devel] [PATCH v4 02/17] qcow2: Pull up overlap check option evaluation, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 01/17] qcow2: Add new overlap check functions, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 03/17] qcow2: Create metadata list, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 04/17] qcow2/overlaps: Protect image header, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 05/17] qcow2/overlaps: Protect refcount table, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 06/17] qcow2/overlaps: Protect refcount blocks, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 07/17] qcow2/overlaps: Protect active L1 table, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 08/17] qcow2/overlaps: Protect active L2 tables, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 09/17] qcow2/overlaps: Protect snapshot table, Max Reitz, 2015/05/04
- [Qemu-devel] [PATCH v4 10/17] qcow2/overlaps: Protect inactive L1 tables, Max Reitz, 2015/05/04