[pbs-devel] [PATCH proxmox-backup 4/4] docs: add description for gc-cache-capacity tuning parameter

[Lukas Wagner]

On  2025-04-04 14:20, Thomas Lamprecht wrote:
> Am 04.04.25 um 13:58 schrieb Lukas Wagner:
>> I think we could completely omit the "the capacity is set as the given value multiplied by 1024" sentence here
>> and consider the fact that the LRU cache size is value * 1024 an implementation detail.
> 
> But once that option is available it's part of the API and not just
> an implementation detail anymore?
> 
>> For the user, the exact number of cached digests in the backend is probably not really that important, right?
>> In reality, they just want some knob that they can adjust in a range from 0 (no caching) to some maximum.
> 
> For some, probably even a lot, users it might be indeed enough to
> like double or half the number depending on if they want to improve
> performance or reduce memory footprint.
> 
> But I know users that have a hard time working with a numerical
> setting without knowing what it's exactly doing on a lower level, at
> least for me such settings are often rather irritating, as I cannot
> really have a good thought process for how I'd choose the number
> depending on what I want to achieve.
> 
> So while I agree with your underlying point, I'd still like users
> being able to relatively easily find out how much change translates
> in what impact.
> 
>> Same of course applies also for the GUI patch and the log message.
>>
>> What do you think?
> 
> Two alternatives:
> - Changes this to the shift width, i.e. the x from 2^x, similar to the
>   ZFS setting. Makes it nice small number to configure and for most
>   use cases the exponential nature should be still granular enough.
>   That said, it's not very user-friendly, at least to those without
>   some level of CS background or the like.
> 
> - just drop the * 1024 factor and allow users to enter the full number,
>   it then can be simply described as numbers of chunks which is trivial
>   to understand and relate too.
> 
> Personally I'd favor the second option, mainly because it's so simple,
> and having big numbers here is not that of a huge problem.

Sounds like a good idea, I like it.
My main gripe with the "times 1024" option was that it makes it a bit
more confusing to the user (e.g. me, when reading 'GC LRU cache capacity (in multiples of 1024 chunk digests)'
in the UI I first thought that the value itself must be a multiple of 1024).

Changing the setting to the full number, we avoid this potential for confusion while
still giving power-users a good sense of what is going on under the hood.

> 
> ps. secret option three: adapt the human byte selector in the frontend
> to expose selecting kilo-chunks and mega-chunks ;-)
> 


-- 
- Lukas