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I wanted to determine if its worth putting ephemeral images into the libvirt cache at all. How expensive are these images to create? They don't need to come from the image service, so it can't be too bad, right? It turns out that qemu-img is very very fast at creating these images, based on the very small data set of my laptop with an ext4 file system... mikal@x220:/data/temp$ time qemu-img create -f raw disk 10g Formatting 'disk', fmt=raw size=10737418240 real 0m0.315s user 0m0.000s sys 0m0.004s mikal@x220:/data/temp$ time qemu-img create -f raw disk 100g Formatting 'disk', fmt=raw size=107374182400 real 0m0.004s user 0m0.000s sys 0m0.000s Perhaps this is because I am using ext4, which does funky extents things when allocating blocks. However, the only ext3 file system I could find at my place is my off site backup disks, which are USB3 attached instead of the SATA2 that my laptop uses. Here's the number from there: $ time qemu-img create -f raw disk 100g Formatting 'disk', fmt=raw size=107374182400 real 0m0.055s user 0m0.000s sys 0m0.004s So still very very fast. Perhaps its the mkfs that's slow? Here's a run of creating a ext4 file system inside that 100gb file I just made on…