qemu/qapi
Alberto Garcia 1221fe6f63 qcow2: Allow configuring the L2 slice size
Now that the code is ready to handle L2 slices we can finally add an
option to allow configuring their size.

An L2 slice is the portion of an L2 table that is read by the qcow2
cache. Until now the cache was always reading full L2 tables, and
since the L2 table size is equal to the cluster size this was not very
efficient with large clusters. Here's a more detailed explanation of
why it makes sense to have smaller cache entries in order to load L2
data:

   https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html

This patch introduces a new command-line option to the qcow2 driver
named l2-cache-entry-size (cf. l2-cache-size). The cache entry size
has the same restrictions as the cluster size: it must be a power of
two and it has the same range of allowed values, with the additional
requirement that it must not be larger than the cluster size.

The L2 cache entry size (L2 slice size) remains equal to the cluster
size for now by default, so this feature must be explicitly enabled.
Although my tests show that 4KB slices consistently improve
performance and give the best results, let's wait and make more tests
with different cluster sizes before deciding on an optimal default.

Now that the cache entry size is not necessarily equal to the cluster
size we need to reflect that in the MIN_L2_CACHE_SIZE documentation.
That minimum value is a requirement of the COW algorithm: we need to
read two L2 slices (and not two L2 tables) in order to do COW, see
l2_allocate() for the actual code.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: c73e5611ff4a9ec5d20de68a6c289553a13d2354.1517840877.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
2018-02-13 17:00:00 +01:00
..
Makefile.objs qapi: rename *qmp-*-visitor* to *qobject-*-visitor* 2016-10-25 16:25:48 +02:00
block-core.json qcow2: Allow configuring the L2 slice size 2018-02-13 17:00:00 +01:00
block.json qapi: add nbd-server-remove 2018-01-26 09:37:20 -06:00
char.json qapi-schema: Collect char device stuff in qapi/char.json 2017-09-04 13:09:12 +02:00
common.json qapi: Create DEFINE_PROP_OFF_AUTO_PCIBAR 2018-02-06 11:08:26 -07:00
crypto.json qapi-schema: Improve section headings 2017-09-04 13:09:12 +02:00
introspect.json qapi-schema: Introspection doc is in the wrong section, fix 2017-09-04 13:09:12 +02:00
migration.json migration: Revert postcopy-blocktime commit set 2018-01-23 10:08:05 +00:00
net.json net: Allow hubports to connect to other netdevs 2018-01-29 16:05:37 +08:00
opts-visitor.c qapi: Fix QemuOpts visitor regression on unvisited input 2017-03-22 19:24:34 +01:00
qapi-clone-visitor.c qapi/qnull: Add own header 2017-11-17 18:21:30 +01:00
qapi-dealloc-visitor.c qdict qlist: Make most helper macros functions 2018-02-09 13:52:15 +01:00
qapi-util.c qapi: Change data type of the FOO_lookup generated for enum FOO 2017-09-04 13:09:13 +02:00
qapi-visit-core.c Include qapi/qmp/qobject.h exactly where needed 2018-02-09 13:52:15 +01:00
qmp-dispatch.c Include qapi/qmp/qdict.h exactly where needed 2018-02-09 13:52:15 +01:00
qmp-event.c Include qapi/qmp/qdict.h exactly where needed 2018-02-09 13:52:15 +01:00
qmp-registry.c qapi: Support multiple command registries per program 2017-03-05 09:12:25 +01:00
qobject-input-visitor.c Include qapi/qmp/qstring.h exactly where needed 2018-02-09 13:52:15 +01:00
qobject-output-visitor.c Include qapi/qmp/qdict.h exactly where needed 2018-02-09 13:52:15 +01:00
rocker.json qapi: The #optional tag is redundant, drop 2017-03-16 07:13:02 +01:00
run-state.json qapi: Rename WatchdogExpirationAction enum 2017-10-02 08:40:01 +02:00
sockets.json qapi-schema: Collect sockets stuff in qapi/sockets.json 2017-09-04 13:09:12 +02:00
string-input-visitor.c qapi/qnull: Add own header 2017-11-17 18:21:30 +01:00
string-output-visitor.c qapi: Use QNull for a more regular visit_type_null() 2017-07-24 13:35:11 +02:00
tpm.json tpm: add CRB device 2018-01-29 14:22:50 -05:00
trace-events qapi: Use QNull for a more regular visit_type_null() 2017-07-24 13:35:11 +02:00
trace.json qapi-schema: Improve section headings 2017-09-04 13:09:12 +02:00
transaction.json qapi-schema: Collect transaction stuff in qapi/transaction.json 2017-09-04 13:09:12 +02:00
ui.json input: add missing JIS keys to virtio input 2018-02-05 19:53:55 -02:00