Add a new migration capability 'fixed-ram'.
The core of the feature is to ensure that each ram page has a specific
offset in the resulting migration stream. The reason why we'd want
such behavior are two fold:
- When doing a 'fixed-ram' migration the resulting file will have a
bounded size, since pages which are dirtied multiple times will
always go to a fixed location in the file, rather than constantly
being added to a sequential stream. This eliminates cases where a vm
with, say, 1G of ram can result in a migration file that's 10s of
GBs, provided that the workload constantly redirties memory.
- It paves the way to implement DIRECT_IO-enabled save/restore of the
migration stream as the pages are ensured to be written at aligned
offsets.
For now, enabling the capability has no effect. The next couple of
patches implement the core funcionality.
Signed-off-by: Fabiano Rosas <faro...@suse.de>
---
docs/devel/migration.rst | 14 ++++++++++++++
migration/options.c | 37 +++++++++++++++++++++++++++++++++++++
migration/options.h | 1 +
migration/savevm.c | 1 +
qapi/migration.json | 5 ++++-
5 files changed, 57 insertions(+), 1 deletion(-)
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst
index c3e1400c0c..6f898b5dbd 100644
--- a/docs/devel/migration.rst
+++ b/docs/devel/migration.rst
@@ -566,6 +566,20 @@ Others (especially either older devices or system devices
which for
some reason don't have a bus concept) make use of the ``instance id``
for otherwise identically named devices.
+Fixed-ram format
+----------------
+
+When the ``fixed-ram`` capability is enabled, a slightly different
+stream format is used for the RAM section. Instead of having a
+sequential stream of pages that follow the RAMBlock headers, the dirty
+pages for a RAMBlock follow its header. This ensures that each RAM
+page has a fixed offset in the resulting migration stream.
+
+The ``fixed-ram`` capaility can be enabled in both source and
+destination with:
+
+ ``migrate_set_capability fixed-ram on``
+
Return path
-----------
diff --git a/migration/options.c b/migration/options.c
index c3def757fe..2622d8c483 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -202,6 +202,7 @@ Property migration_properties[] = {
DEFINE_PROP_MIG_CAP("x-dirty-limit", MIGRATION_CAPABILITY_DIRTY_LIMIT),
DEFINE_PROP_BOOL("x-auto-pause", MigrationState,
capabilities[MIGRATION_CAPABILITY_AUTO_PAUSE], true),
+ DEFINE_PROP_MIG_CAP("x-fixed-ram", MIGRATION_CAPABILITY_FIXED_RAM),
DEFINE_PROP_END_OF_LIST(),
};
@@ -268,6 +269,16 @@ bool migrate_events(void)
return s->capabilities[MIGRATION_CAPABILITY_EVENTS];
}
+bool migrate_fixed_ram(void)
+{
+/*
+ MigrationState *s = migrate_get_current();
+
+ return s->capabilities[MIGRATION_CAPABILITY_FIXED_RAM];
+*/
+ return false;
+}
+
bool migrate_ignore_shared(void)
{
MigrationState *s = migrate_get_current();
@@ -627,6 +638,32 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps,
Error **errp)
}
}
+ if (new_caps[MIGRATION_CAPABILITY_FIXED_RAM]) {
+ if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) {
+ error_setg(errp,
+ "Fixed-ram migration is incompatible with multifd");
+ return false;
+ }
+
+ if (new_caps[MIGRATION_CAPABILITY_XBZRLE]) {
+ error_setg(errp,
+ "Fixed-ram migration is incompatible with xbzrle");
+ return false;
+ }
+
+ if (new_caps[MIGRATION_CAPABILITY_COMPRESS]) {
+ error_setg(errp,
+ "Fixed-ram migration is incompatible with compression");
+ return false;
+ }
+
+ if (new_caps[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
+ error_setg(errp,
+ "Fixed-ram migration is incompatible with postcopy
ram");
+ return false;
+ }
+ }
+
return true;
}
diff --git a/migration/options.h b/migration/options.h
index d1ba5c9de7..2a9e0e9e13 100644
--- a/migration/options.h
+++ b/migration/options.h
@@ -32,6 +32,7 @@ bool migrate_compress(void);
bool migrate_dirty_bitmaps(void);
bool migrate_dirty_limit(void);
bool migrate_events(void);
+bool migrate_fixed_ram(void);
bool migrate_ignore_shared(void);
bool migrate_late_block_activate(void);
bool migrate_multifd(void);
diff --git a/migration/savevm.c b/migration/savevm.c
index 8622f229e5..54e084122a 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -243,6 +243,7 @@ static bool should_validate_capability(int capability)
/* Validate only new capabilities to keep compatibility. */
switch (capability) {
case MIGRATION_CAPABILITY_X_IGNORE_SHARED:
+ case MIGRATION_CAPABILITY_FIXED_RAM:
return true;
default:
return false;
diff --git a/qapi/migration.json b/qapi/migration.json
index 74f12adc0e..1317dd32ab 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -527,6 +527,9 @@
# VM before migration for an optimal migration performance.
# Enabled by default. (since 8.1)
#
+# @fixed-ram: Migrate using fixed offsets for each RAM page. Requires
+# a seekable transport such as a file. (since 8.1)
+#
# Features:
#
# @unstable: Members @x-colo and @x-ignore-shared are experimental.
@@ -543,7 +546,7 @@
{ 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
'validate-uuid', 'background-snapshot',
'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
- 'dirty-limit', 'auto-pause'] }
+ 'dirty-limit', 'auto-pause', 'fixed-ram'] }
##
# @MigrationCapabilityStatus:
--
2.35.3
