When the synchronous read and write functions were dropped, they were replaced
by generic emulation functions. Unfortunately, these emulation functions don't
provide the same semantics as the original functions did.
The original bdrv_read would mean that we read some data synchronously and that
we won't be interrupted during this read. The latter assumption is no longer
true with the emulation function which needs to use qemu_aio_poll and therefore
allows the callback of any other concurrent AIO request to be run during the
read. Which in turn means that (meta)data read earlier could have changed and
be invalid now. qcow2 is not prepared to work in this way and it's just scary
how many places there are where other requests could run.
I'm not sure yet where exactly it breaks, but you'll see breakage with virtio
on qcow2 with a backing file. Providing synchronous functions again fixes the
problem for me.
Patchworks-ID: 35437
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Problem: Our file sys-queue.h is a copy of the BSD file, but there are
some additions and it's not entirely compatible. Because of that, there have
been conflicts with system headers on BSD systems. Some hacks have been
introduced in the commits 15cc923584,
f40d753718,
96555a96d7 and
3990d09adf but the fixes were fragile.
Solution: Avoid the conflict entirely by renaming the functions and the
file. Revert the previous hacks.
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
When two AIO requests write to the same cluster, and this cluster is
unallocated, currently both requests allocate a new cluster and the second one
merges the first one when it is completed. This means an cluster allocation, a
read and a cluster deallocation which cause some overhead. If we simply let the
second request wait until the first one is done, we improve overall performance
with AIO requests (specifially, qcow2/virtio combinations).
This patch maintains a list of in-flight requests that have allocated new
clusters. A second request touching the same cluster is limited so that it
either doesn't touch the allocation of the first request (so it can have a
non-overlapping allocation) or it waits for the first request to complete.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
The wrong version of the preallocation patch has been applied, so this is the
remaining diff.
We can't use truncate to grow the image file to the right size because we don't
know if metadata has been written after the last data cluster. In this case
truncate would shrink the file and destroy its metadata. Write a zero sector at
the end of the virtual disk instead to ensure that the file is big enough.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This introduces a qemu-img create option for qcow2 which allows the metadata to
be preallocated, i.e. clusters are reserved in the refcount table and L1/L2
tables, but no data is written to them. Metadata is quite small, so this
happens in almost no time.
Especially with qcow2 on virtio this helps to gain a bit of performance during
the initial writes. However, as soon as create a snapshot, we're back to the
normal slow speed, obviously. So this isn't the real fix, but kind of a cheat
while we're still having trouble with qcow2 on virtio.
Note that the option is disabled by default and needs to be specified
explicitly using qemu-img create -f qcow2 -o preallocation=metadata.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
The VM state offset is a concept internal to the image format. Replace
the old bdrv_{get,put}_buffer method that require an index into the
image file that is constructed from the VM state offset and an offset
into the vmstate with the bdrv_{load,save}_vmstate that just take an
offset into the VM state.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Contrary to what one could expect, the size of L1 tables is not cluster
aligned. So as we're writing whole sectors now instead of single entries,
we need to ensure that the L1 table in memory is large enough; otherwise
write would access memory after the end of the L1 table.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
The performance of qcow2 has improved meanwhile, so we don't need to
special-case it any more. Switch the default to write-through caching
like all other block drivers.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
The qcow2 source is now split into several more manageable files. During the
conversion quite some functions that were static before needed to be changed to
be global to make the source compile again.
We were lucky enough not to get name conflicts with these additional global
names, but they are not nice. This patch adds a qcow2_ prefix to all of the
global functions in qcow2.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
qcow2-snapshot.c contains the code related to snapshotting.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
qcow2-cluster.c contains all functions related to the management of guest
clusters, i.e. what the guest sees on its virtual disk. This code is about
mapping these guest clusters to host clusters in the image file using the
two-level lookup tables.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
qcow2-refcount.c contains all functions which are related to cluster
allocation and management in the image file. A large part of this is the
reference counting of these clusters.
Also a header file qcow2.h is introduced which will contain the interface of
the split qcow2 modules.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Larger cluster sizes mean less metadata. This has been discussion a few times,
let's do it now. This turns 64k clusters on by default for new images.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This patch adds a small help text to each of the options in the block drivers
which can be displayed by using qemu-img create -f fmt -o ?
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Now that we have a separate aio pool structure we can remove those
aio pool details from BlockDriver.
Every driver supporting AIO now needs to declare a static AIOPool
with the aiocb size and the cancellation method. This cleans up the
current code considerably and will make it cleaner and more obvious
to support two different aio implementations behind a single
BlockDriver.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Don't write each single changed refcount block entry to the disk after it is
written, but update all entries of the block and write all of them at once.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This is a preparation patch with no functional changes. It moves the allocation
of new refcounts block to a new function and makes update_cluster_refcount (for
one cluster) call update_refcount (for multiple clusters) instead the other way
round.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
There is only one (internal) user left and it can be switched to the normal
emulation provided in block.c
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Add an option to specify the cluster size of a newly created qcow2 image.
Default is 4k which is the same value that was hard-coded before.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Now we can make use of the newly introduced option structures. Instead of
having bdrv_create carry more and more parameters (which are format specific in
most cases), just pass a option structure as defined by the driver itself.
bdrv_create2() contains an emulation of the old interface to simplify the
transition.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
