On Thu, Feb 23, 2012 at 11:01:46AM +0530, PANKAJ RAWAT wrote: > I 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 ( during expansion of qcow2 image)
It's not true that performance should increase by raising the cluster size, otherwise the default would be infinity. It's an algorithms/data structure trade-off. Most importantly is the relative latency between a small guest I/O request (e.g. 4 KB) and the cluster size (e.g. 64 KB). If the cluster size latency is orders of magnitude larger than a small guest I/O request, then be prepared to see extreme effects described below: * Bigger clusters decrease the frequency of metadata operations and increase metadata cache hit rates. Bigger clusters means less metadata so qcow2 performs fewer metadata operations overall. Performance boost. * Bigger clusters increase the cost of allocating a new cluster. For example, a 8 KB write to a new cluster will incur a 1 MB write to the image file (the untouched regions are filled with zeros). This can be optimized in some cases but not everywhere (e.g. reallocating a data cluster versus extending the image file size and relying on the file system to provide zeroed space). This is especially expensive when a backing file is in use because up to 1 MB of the backing file needs to be read to populate the newly allocated cluster! Performance loss. * Bigger clusters can reduce fragmentation of data on the physical disk. The file system sees fewer, bigger allocating writes and is therefore able to allocate more contiguous data - less fragmentation. Performance boost. * Bigger clusters reduce the compactness of sparse files. you use more image file space on the host file system when the cluster size is large. Space efficiency loss. Here's a scenario where a 1 MB cluster size is great compared to a large cluster size: You have a fully allocated qcow2 image, you will never need to do any allocating writes. Here's a scenario where a 1 MB cluster size is terrible compared to a small cluster size: You have an empty qcow2 file and perform 4 KB writes to the first sector of each 1 MB chunk, and there is a backing file. So it depends on the application. Stefan