[Qemu-devel] [PATCH 0/7] qcow2: Reduce the number of I/O ops when doing COW

[Alberto Garcia <berto@igalia.com>]

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