Hi,
Now librbd is the only user for set_alloc_hint op.
void AioWrite::add_write_ops(librados::ObjectWriteOperation &wr) {
wr.set_alloc_hint(m_ictx->get_object_size(), m_ictx->get_object_size());
wr.write(m_object_off, m_write_data);
}
According above, the arguments for set_alloc_hint both are the size of
a object. It looks like there exist a misleading of the argument name
called "expected_write_size". And I would like to think
"expected_write_size" as the write size for most write ops.
KeyValueStore in recent commits use "expected_write_size" as strip
size, unfortunately librbd will issue set_alloc_hint(4MB, 4MB) but
each write op's size is serval KB.
I'm not sure "expected_object_size" and "expected_write_size" for
set_alloc_hint are properly used by librbd or maybe I misunderdstand
the op.
As for FileStore, if use XFS, set_alloc_hint will let FS allocate in
multiples of the hint size for this file. And some users may use 16MB
as the order of librbd image(like me :-| ), so it seemed a little
space ineffective.
I pushed a fix for keyvaluestore to avoid using
set_alloc_op(https://github.com/ceph/ceph/pull/1967).
--
Best Regards,
Wheat
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