On 6/4/21 7:07 PM, Santiago Castillo Oli wrote: > Hi all! > > > I'm using bcache and I think I have a rather low hit ratio and cache > occupation. > > > My setup is: > > - Cache device: 82 GiB partition on a SSD drive. Bucket size=4M. The > partition is aligned on a Gigabyte boundary. > > - Backing device: 3.6 TiB partition on a HDD drive. There is 732 GiB > of data usage on this partition. This 732 GiB are used by 9 qcow2 > files assigned to 3 VMs running on the host. > > - Neither the SDD nor HDD drives have another partitions in use. > > - After 24 hours of use, according to priority_stats the cache is 75% > Unused (63 GiB Unused - 19 GiB used), but... > > - ... according to "smartctl -a" in those 24 hours "Writes to Flash" > has increased in 160 GiB and "GB written from host" has increased in > 90 GiB > > - cache_hit_ratio is 10 % > > > > - I'm using maximum bucket size (4M) trying to minimize write > amplification. With this bucket size, "Writes to Flash" (160) to "GB > written from host"(90) ratio is 1,78. Previously, some days ago, I was > using default bucket size. The write amplification ratio then was 2,01. > > - Isn't the cache_hit_ratio (10%) a bit low? > > - Is it normal that, after 24 hours running, the cache occupation is > that low (82-63 = 19GiB, 25%) when the host has written 90 GiB to the > cache device in the same period? I don´t understand why 90 GiB of data > has been written to fill 19 GiB of cache. > > > Any ideas? > > > Thank you and regards. > > What is the kernel version and where do you have the kernel ? And what is the workload on your machine ? Most of the read requests are missing, so they will read from backing device and refilled into cache device as used-and-clean data. Once there is no enough space to hold more read-cached data, garbage colleague may retire the used-and-clean data very fast and make available room for new refilling read data. The 19GB data might be existing data from last time gc. Coly Li
