[pbs-devel] [PATCH proxmox-backup v6 29/37] datastore: add local datastore cache for network attached storages

[Thomas Lamprecht]

Am 08.07.25 um 19:01 schrieb Christian Ebner:
> Use a local datastore as cache using LRU cache replacement policy for
> operations on a datastore backed by a network, e.g. by an S3 object
> store backend. The goal is to reduce number of requests to the
> backend and thereby save costs (monetary as well as time).
> 
> The cacher allows to fetch cache items on cache misses via the access
> method.

FWIW, this might be also be used for local data stores backed by slower
storage as an application aware–and thus more efficient–caching mechanism.
E.g., for those setups with huge and slow storage for actual backup pools
to setup the cache on a smaller but faster (e.g. flash based) storage.

But nothing we need to incorporate as part of this series, just an idea
to potentially investigate after this landed.

Some before/after benchmark and words about chosen cache size might be
nice here too, again just to get a rough ballpark estimation about what
order of magnitude of change one might get from this.

Also, just from reading the commit message it's not entirely clear to me
on which medium/path the data will be actually cached on, while that is
pretty much given from the current core design of saving the indexes to
the local storage anyway, it still might be nice to mention that
explicitly here.

> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index c1ba2dcea..ef146e84a 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs

> @@ -437,6 +471,16 @@ impl DataStore {
>                  .parse_property_string(config.backend.as_deref().unwrap_or(""))?,
>          )?;
>  
> +        const LOCAL_DATASTORE_CACHE_SIZE: usize = 10_000_000;

This is amount of chunks or?

The cache size could be derived, or at least limited from available free
space of the local datastores backing storage, for a cache that is backed
on storage being able to configure the limits will be probably good to
have as tuning knob for admins (providing Somewhat OK™ default value is
still great to avoid making tuning a necessity in common setups).


> +        let lru_store_caching = if DatastoreBackendType::S3 == backend_config.ty.unwrap_or_default()
> +        {
> +            let cache =
> +                LocalDatastoreLruCache::new(LOCAL_DATASTORE_CACHE_SIZE, chunk_store.clone());
> +            Some(cache)
> +        } else {
> +            None
> +        };
> +
>          Ok(DataStoreImpl {
>              chunk_store,
>              gc_mutex: Mutex::new(()),