Hi all, here's a patch series that rewrites the copy-on-write code in the qcow2 driver to reduce the number of I/O operations.
The situation is that when a guest sends a write request and QEMU needs to allocate new cluster(s) in a qcow2 file, the unwritten regions of the new cluster(s) need to be filled with the existing data (e.g. from the backing image) or with zeroes. The whole process can require up to 5 I/O operations: 1) Write the data from the actual write request. 2) Read the existing data located before the guest data. 3) Write that data to the new clusters. 4) Read the existing data located after the guest data. 5) Write that data to the new clusters. This series reduces that to only two operations: 1) Read the existing data from the original clusters 2) Write the updated data (=original + guest request) to the new clusters Step (1) implies that there's data that will be read but will be immediately discarded (because it's overwritten by the guest request). I haven't really detected any big performance problems because of that, but I decided to be conservative and my code includes a simple heuristic that keeps the old behavior if the amount of data to be discarded is higher than 16KB. I've been testing this series in several scenarios, with different cluster sizes (32K, 64K, 1MB) and request sizes (from 4 up to 512KB), and both with an SSD and a rotating HDD. The results vary depending on the case, with an average increase of 60% in the number of IOPS in the HDD case, and 15% in the SSD case. In some cases there are really no big differences and the results are similar before and after this patch. Further work for the future includes detecting when the data that needs to be written consists on zeroes (i.e. allocating a new cluster with no backing image) and optimizing that case, but let's start with this. Regards, Berto Alberto Garcia (7): qcow2: Remove unused Error in do_perform_cow() qcow2: Use unsigned int for both members of Qcow2COWRegion qcow2: Make perform_cow() call do_perform_cow() twice qcow2: Split do_perform_cow() into _read(), _encrypt() and _write() qcow2: Allow reading both COW regions with only one request qcow2: Pass a QEMUIOVector to do_perform_cow_{read,write}() qcow2: Merge the writing of the COW regions with the guest data block/qcow2-cluster.c | 188 +++++++++++++++++++++++++++++++++++++------------- block/qcow2.c | 58 +++++++++++++--- block/qcow2.h | 11 ++- 3 files changed, 197 insertions(+), 60 deletions(-) -- 2.11.0