[Libguestfs] Effect of qemu-img convert -m and -W options

[CC to qemu-devel since I'm obviously doing something wrong here, I'm just not sure what.] I was getting ready to add multiple threads to ‘qemu-img convert’ (the longest part of v2v conversions) when I noticed that it had them already! (To be fair this was only added in February this year so no wonder we didn't notice.) To enable parallel convert we would need to use the ‘qemu-img convert -W’ option (which allows out-of-order writes to the destination) and ‘qemu-img convert -m <#num-coroutines>’ option to select the parallelism (defaults to 8). The documentation refers to coroutines but I verified from strace that it is using real threads. I did some testing to see what effect this has. For this I used a large guest image which is approximately a third full of random data (the rest being sparsely allocated): Source format: raw Source virtual size: 100 GB Source used space: 31 GB Target format: qcow2 Version: qemu-img-2.10.0-7.fc28.x86_64 Conversion command: rm -f /to/target time qemu-img convert [-W] [-m ##] -f raw source.img -O qcow2 /to/target Source and target are regular files on two different disks. The test machine is a Xeon E5 with 16 real cores. ---------------------------------------------------------------------- Non-preallocated output (times are in seconds) without -W -W -m 1 153 - -m 4 155 157 -m 8 [default] 158 231 -m 16 [max] 166 166 ---------------------------------------------------------------------- The documentation for ‘-W’ notes that this is only recommended for preallocated outputs (which the test above does not use). So let's try using a preallocated qcow2 output. Conversion command: # the same target file is reused each time time qemu-img convert -n [-W] [-m ##] -f raw source.img -O qcow2 /to/target ---------------------------------------------------------------------- Preallocated output (times are in seconds) without -W -W -m 1 147 - -m 4 146 145 -m 8 [default] 146 199 -m 16 [max] 147 146 ---------------------------------------------------------------------- Based on this there seems to be some issue with the ‘-W’ option -- I even thought I might have it backwards, but checking the code it does seem like ‘-W’ enables (rather than disables) out of order writes. Also some bizarre interaction between ‘-W’ and ‘-m 8’. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org