Hello! Am Fr., 4. Juni 2021 um 14:36 Uhr schrieb Santiago Castillo Oli <scastillo@xxxxxxxxx>: > > Hi Coli! > > > El 04/06/2021 a las 14:05, Coly Li escribió: > > What is the kernel version and where do you have the kernel ? And what > > is the workload on your machine ? > > I'm using debian 10 with default debian kernel (4.19.0-16-amd64) in host > and guests. > > For virtualization I'm using KVM. > > > There is a host, where bcache is running. The filesystem over bcache > device is ext4. In that filesystem there is only 9 qcow2 files user by > three VM guests. Two VM are running small nextcloud instances, another > one is running transmission (bittorrent) for feeding debian and other > distro iso files (30 files - 60 GiB approx.) Besides Coly recommending to use a newer kernel, I think there may be some misunderstanding of how bcache works: * bcache is mostly about reducing latency so it skips sequential access, you should measure block access latency instead of throughput or fill rate * thus, it probably fills your cache very slowly if a lot of patterns are sequential * ext4 has a write journal which turns many random write patterns into sequential write patterns, YMMV if you disable ordered data mode or journalling * qcow2 is copy-on-write: new blocks are appended, resulting in possible write amplification in bcache, it also creates sequential write patterns * kvm/qemu probably use directio / uncached IO by default which may bypass bcache or caching completely, you should try a different IO mode in kvm (e.g. unsafe cached) Regards, Kai
