[Qemu-devel] effect of cluster_size on qcow2 performance during expansion

[PANKAJ RAWAT]

Hi all

I have been working on the qcow2 image format ,

In theory regarding  cluster size it is written that as the size of cluster increase performance should increase.

 

But something surprising happen The performance is degrading as the size of cluster increased from 64K to 1M or 2M  ( during expansion of qcow2 image)

 

By expansion i mean that initially qcow2 image is small in size and size increases when we perform any write operation on it.

Here are the reading taken through fio tool during write operation

Cluster size : 32K    587Mb/sec (aggrb MB/sec)

Cluster size : 64K    697Mb/sec (aggrb MB/sec)

Cluster size : 1M     440Mb/sec 
(aggrb MB/sec)

Cluster size : 2M     289Mb/sec  (aggrb MB/sec)

as in above it can bee seen that the 64k is performing better

can any one tell why ?

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Pankaj Rawat