The current algorithm to replace entries from the L2 cache gives
priority to newer hits by dividing the hit count of all existing
entries by two everytime there is a cache miss.
However, if there are several cache misses the hit count of the
existing entries can easily go down to 0. This will result in those
entries being replaced even when there are others that have never been
used.
This problem is more noticeable with larger disk images and cache
sizes, since the chances of having several misses before the cache is
full are higher.
If we make sure that the hit count can never go down to 0 again,
unused entries will always have priority.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
With a variable cache size, the number given to qcow2_cache_create() may
be huge. Therefore, use g_try_new0().
While at it, use g_new0() instead of g_malloc0() for allocating the
Qcow2Cache object.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
g_new(T, n) is safer than g_malloc(sizeof(*v) * n) for two reasons.
One, it catches multiplication overflowing size_t. Two, it returns
T * rather than void *, which lets the compiler catch more type
errors.
Perhaps a conversion to g_malloc_n() would be neater in places, but
that's merely four years old, and we can't use such newfangled stuff.
This commit only touches allocations with size arguments of the form
sizeof(T), plus two that use 4 instead of sizeof(uint32_t). We can
make the others safe by converting to g_malloc_n() when it becomes
available to us in a couple of years.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Some code in the block layer makes potentially huge allocations. Failure
is not completely unexpected there, so avoid aborting qemu and handle
out-of-memory situations gracefully.
This patch addresses the allocations in the qcow2 block driver.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
In qcow2_check_metadata_overlap and qcow2_pre_write_overlap_check,
change the parameter signifying the checks to perform from its current
positive form to a negative one, i.e., it will no longer explicitly
specify every check to perform but rather a mask of checks not to
perform.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Add a function for emptying a cache, i.e., flushing it and marking all
elements invalid.
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
The pre-write overlap check function is now called before most of the
qcow2 writes (aborting it on collision or other error).
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Writethrough does not need special-casing anymore in the qcow2 caches.
The block layer adds flushes after every guest-initiated data write,
and these will also flush the qcow2 caches to the OS.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
In snapshotting there is no guest involved, so we can safely use a writeback
mode and do the flushes in the right place (i.e. at the very end). This
improves the time that creating/restoring an internal snapshot takes with an
image in writethrough mode.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Move size after the two pointers in struct Qcow2Cache to get better
packing of struct elements on 64 bit architectures.
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
qcow2 calls bdrv_flush() after performing COW in order to ensure that the
L2 table change is never written before the copy is safe on disk. Now that the
L2 table is cached, we can wait with flushing until we write out the next L2
table.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This adds some new cache functions to qcow2 which can be used for caching
refcount blocks and L2 tables. When used with cache=writethrough they work
like the old caching code which is spread all over qcow2, so for this case we
have merely a cleanup.
The interesting case is with writeback caching (this includes cache=none) where
data isn't written to disk immediately but only kept in cache initially. This
leads to some form of metadata write batching which avoids the current "write
to refcount block, flush, write to L2 table" pattern for each single request
when a lot of cluster allocations happen. Instead, cache entries are only
written out if its required to maintain the right order. In the pure cluster
allocation case this means that all metadata updates for requests are done in
memory initially and on sync, first the refcount blocks are written to disk,
then fsync, then L2 tables.
This improves performance of scenarios with lots of cluster allocations
noticably (e.g. installation or after taking a snapshot).
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
