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

[Alberto Garcia <berto@igalia.com>]

ping

On Tue, May 23, 2017 at 01:22:55PM +0200, Alberto Garcia wrote:
> 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