On Mon, Mar 23, 2020 at 2:57 AM Zdenek Kabelac <zkabelac@xxxxxxxxxx> wrote: > Dne 23. 03. 20 v 9:26 Joe Thornber napsal(a): > > On Sun, Mar 22, 2020 at 10:57:35AM -0700, Scott Mcdermott wrote: > > > have a 931.5 GibiByte SSD pair in raid1 (mdraid) as cache LV for a > > > data LV on 1.8 TebiByte raid1 (mdraid) pair of larger spinning disk. > > Users should be 'performing' some benchmarking about the 'useful' sizes of > hotspot areas - using nearly 1T of cache for 1.8T of origin doesn't look > the right ration for caching. > (i.e. like if your CPU cache would be halve of your DRAM) the 1.8T origin will be upgraded over time with larger/more spinning disks, but the cache will remain as it is. hopefully it can perform well whether it is 1:2 cache:data as now or 1:10+ as later. > Too big 'cache size' leads usually into way too big caching chunks > (since we try to limit number of 'chunks' in cache to 1 milion - you > can rise up this limit - but it will consume a lot of your RAM space as well) > So IMHO I'd recommend to use at most 512K chunks - which gives you > about 256GiB of cache size - but still users should be benchmarking what is > the best for them...) how to raise this limit? since I'm low RAM this is a problem, but why are large chunks an issue, besides memory usage? is this causing unnecessary I/O by an amplification effect? if my system doesn't have enough memory for this job I will have to find a host board with more RAM. > Another hint - lvm2 introduced support for new dm-writecache target as well. this won't work for me since a lot of my data is reads, and I'm low memory with large numbers of files. rsync of large trees is the main workload; existing algorithm is not working fantastically well, but nonetheless giving a nice boost to my rsync completion times over the uncached times. _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
