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[libvirt] speeding up qemu core dumps



Hi All,

I've had user reports of "libvirt limiting KVM core dumps".   This isn't
quite true since libvirt just uses the default migration speed defined
in qemu's migration.c

/* Migration speed throttling */
static int64_t max_throttle = (32 << 20);

AFAIK, there's not much interest in changing the qemu default

http://lists.nongnu.org/archive/html/qemu-devel/2011-02/msg01156.html

I thought there were more discussions on the topic but can only find the
above thread ATM.

Do folks here have objections to increasing the migration speed prior to
core dump?  If not, any suggestions for a value?  I set the migration
speed to 1G in doCoreDump() prior to calling qemuMigrationToFile() with
following improvement dumping a 4G guest

32MiB/s migration speed (qemu default):
# time virsh dump --live test /tmp/dump
Domain test dumped to /tmp/dump

real 0m10.121s
user 0m0.004s
sys 0m0.004s

1GiB/s migration speed:
# time virsh dump --live test /tmp/dump
Domain test dumped to /tmp/dump

real 0m3.745s
user 0m0.004s
sys 0m0.004s

I suppose the same applies for save.

Regards,
Jim


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