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I have done some tests with suballocated cluster and base image without
backing_file, indeed, I'm seeing a small performance degradation on big
1TB image.
with a 30GB image, I'm around 22000 iops 4k randwrite/randread (with
or without l2_extended=on)
with a 1TB image, the result is different
fio –filename=/dev/sdb –direct=1 –rw=randwrite –bs=4k –iodepth=32
–ioengine=libaio –name=test
default l2-cache-size , extended_l2=off, cluster_size=64k : 2700 iops
default l2-cache-size , extended_l2=on, cluster_size=128k: 1500 iops
I have also play with qemu l2-cache-size option of drive (default value
is 1MB, and it's not enough for a 1TB image to keep all metadatas in
memory)
l2-cache-size=8MB , extended_l2=off, cluster_size=64k: 2900 iops
l2-cache-size=64MB , extended_l2=off, cluster_size=64k: 5100 iops
l2-cache-size=128MB , extended_l2=off, cluster_size=64k : 22000 iops
l2-cache-size=8MB , extended_l2=on, cluster_size=128k: 2000 iops
l2-cache-size=64MB , extended_l2=on, cluster_size=128k: 4500 iops
l2-cache-size=128MB , extended_l2=on, cluster_size=128k: 22000 iops
So no difference in needed memory, with or with extended_l2.
but the l2-cache-size tuning is really something we should add in
another patch I think ,for general performance with qcow2.
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