On August 13, 2025 3:46 pm, Alexandre Derumier via pve-devel wrote: >Currently, the qcow2 maximum cache-size is 32MB, enough to handle metadatas >for a 256GB images with 65k cluster size. > >if cache-size is not big enough, performance are really bad > >This patch increase cache to 1GB, enough to handle 8TB image > >with default 32MB cache >fio benchmark 4k randread/write: > >256GB image : 32MB cache : 40000 iops >1TB image: 32MB cache: 2500 iops >8TB image: 32MB cache: 2500 iops >1TB image: 1G cache: 40000 iops >8TB image: 1G cache: 40000 iops > >the cache-size value is a maximum value, memory is not reserved at vm start. >Also, the metadatas not used are removed from the cache after 10min by defa= >ult. > >https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt >Signed-off-by: Alexandre Derumier <alexandre.derum...@groupe-cyllene.com> >--- > src/PVE/QemuServer/Blockdev.pm | 3 +++ > src/test/cfg2cmd/bootorder-empty.conf.cmd | 6 +++--- > src/test/cfg2cmd/bootorder-legacy.conf.cmd | 6 +++--- > src/test/cfg2cmd/bootorder.conf.cmd | 6 +++--- > .../cfg2cmd/cputype-icelake-client-deprecation.conf.cmd | 2 +- > src/test/cfg2cmd/ide.conf.cmd | 2 +- > src/test/cfg2cmd/q35-ide.conf.cmd | 2 +- > src/test/cfg2cmd/q35-linux-hostpci-mapping.conf.cmd | 2 +- > src/test/cfg2cmd/q35-linux-hostpci-multifunction.conf.cmd | 2 +- > src/test/cfg2cmd/q35-linux-hostpci-template.conf.cmd | 2 +- > src/test/cfg2cmd/q35-linux-hostpci-x-pci-overrides.conf.cmd | 2 +- > src/test/cfg2cmd/q35-linux-hostpci.conf.cmd | 2 +- > src/test/cfg2cmd/q35-simple.conf.cmd | 2 +- > src/test/cfg2cmd/seabios_serial.conf.cmd | 2 +- > src/test/cfg2cmd/simple-backingchain.conf.cmd | 4 ++-- > src/test/cfg2cmd/simple-virtio-blk.conf.cmd | 2 +- > src/test/cfg2cmd/simple1-template.conf.cmd | 4 ++-- > src/test/cfg2cmd/simple1.conf.cmd | 2 +- > src/test/run_qemu_img_convert_tests.pl | 6 +++--- > 19 files changed, 31 insertions(+), 28 deletions(-) > >diff --git a/src/PVE/QemuServer/Blockdev.pm b/src/PVE/QemuServer/Blockdev.pm >index f5798cbf..895f1a6a 100644 >--- a/src/PVE/QemuServer/Blockdev.pm >+++ b/src/PVE/QemuServer/Blockdev.pm >@@ -381,6 +381,9 @@ my sub generate_format_blockdev { > $blockdev->{'discard-no-unref'} =3D JSON::true if $format eq 'qcow= >2'; > } >=20 >+ #increase qcow2 maximum cache size to 1GB memory, enough to handle 8TB= > disk image with 64k clusters. >+ $blockdev->{'cache-size'} =3D 1073741824 if $format eq 'qcow2'; >+ > return $blockdev; > }
have you benchmarked this? if so, did you compare it with using the smaller cache-entry variant described in the file you linked: https://gitlab.com/qemu-project/qemu/-/blob/master/docs/qcow2-cache.txt#L159 we also know the image size here, so we could use a capped, derived value? what if the disk is resized? what about image files with bigger clusters? _______________________________________________ pve-devel mailing list pve-devel@lists.proxmox.com https://lists.proxmox.com/cgi-bin/mailman/listinfo/pve-devel
