Re: [9front] vm disk performance

[Xiao-Yong Jin <meta.jxy@gmail.com>]

> On Apr 15, 2021, at 4:52 AM, hiro <23hiro@gmail.com> wrote:
> 
> i realized that my 9front KVM cwfs install assumes 512byte sectors.
> the physical disk actually uses 4k sectors.
> 
> the partition table also aligns things by sector size.
> until nvram we are aligned by big enough multiples of 512 that they
> happen to also be multiples of 4k.
> but nvram is one sector big, causing the next important partition
> (other) to be misaligned by those 512bytes.
> 
> these 2 things together might affect performance considerably,
> especially on hdds.
> 
> personally i also use zvols from zfs between the VMs and my disk, they
> also happen to have a blocksize, the rather low default should
> generally be increased to 16k to align with cwfs. even though i doubt
> anybody else here is using zvols :)

% zfs list -o space,type,compress,compressratio,volmode,volsize,volblocksize zroot/9front      
NAME          AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD  TYPE    COMPRESS  RATIO  VOLMODE  VOLSIZE  VOLBLOCK
zroot/9front   353G  45.9G     5.96G   15.1G          24.9G          0  volume       lz4  1.46x      dev      30G        8K

> i remember also a long time ago i discovered (on shady forums, don't
> remember) that kvm on linux by default forces all normal (async)
> writes to the virtual disk for "legacy" operating systems to by
> synchronous to protect us from ourselves.
> personally i disabled sync writes on my zfs as a good enough remedy,
> but i have *no clue* how much better it might work if kvm could be
> convinced we are a microsoft/redhat certified non-legacy system.
> 
> since i managed to find already 4 very probable big bottlenecks for
> common setups with 9front inside VMs, i'd like to share my preliminary
> view on this matter.
> 
> tests outstanding.

I never worried about performance here, because network latency dominates
most of the use cases.  Perhaps you may produce some benchmarks for a
couple of use cases in mind?