[pbs-devel] [PATCH proxmox-backup 4/7] chunk store: fix: replace evicted cache chunks instead of truncate

[Christian Ebner]

On 10/6/25 3:18 PM, Fabian Grünbichler wrote:
> On October 6, 2025 12:41 pm, Christian Ebner wrote:
>> Evicted chunks have been truncated to size zero, keeping the chunk
>> file in place as in-use marker for the garbage collection but freeing
>> the chunk file contents. This can however lead to restores failing if
>> they already opened the chunk file for reading, as their contents are
>> now incomplete.
>>
>> Fix this by instead replacing the chunk file with a zero sized file,
>> leaving the contents accessible for the already opened chunk readers.
>>
>> By moving the logic from the local datastore cache to a helper method
>> on the chunk store, it is also assured that the operation is guarded
>> by the chunk store mutex lock to avoid races with chunk re-insert.
> 
> AFAICT this is still racy even after this patch, because in cache.access
> after a cache miss (either no local file, or empty local file) we fetch
> the chunk from S3, insert it into the chunk store (cache), but then
> instead of returning the chunk from the already in-memory chunk data, we
> load it again from the path - without holding the lock that prevents the
> chunk from being evicted again.. while unlikely to be hit in practice,
> this is still wasteful because we could save a round-trip and just
> return the chunk we've already constructed from the S3 response and hit
> two birds with one stone?

Good catch! Indeed we can serve the response from the already in-memory 
data in that case.

> 
>>
>> Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
>> ---
>>   pbs-datastore/src/chunk_store.rs              | 20 +++++++++++++++
>>   .../src/local_datastore_lru_cache.rs          | 25 +++----------------
>>   2 files changed, 23 insertions(+), 22 deletions(-)
>>
>> diff --git a/pbs-datastore/src/chunk_store.rs b/pbs-datastore/src/chunk_store.rs
>> index 010785fbc..74fa79db1 100644
>> --- a/pbs-datastore/src/chunk_store.rs
>> +++ b/pbs-datastore/src/chunk_store.rs
>> @@ -668,6 +668,26 @@ impl ChunkStore {
>>           (chunk_path, digest_str)
>>       }
>>   
>> +    /// Replace a chunk file with a zero size file in the chunk store.
>> +    ///
>> +    /// Used to evict chunks from the local datastore cache, while keeping them as in-use markers
>> +    /// for garbage collection. Returns with success also if chunk file is not pre-existing.
>> +    pub fn clear_chunk(&self, digest: &[u8; 32]) -> Result<(), Error> {
>> +        let (chunk_path, digest_str) = self.chunk_path(digest);
>> +        let mut create_options = CreateOptions::new();
>> +        if nix::unistd::Uid::effective().is_root() {
>> +            let uid = pbs_config::backup_user()?.uid;
>> +            let gid = pbs_config::backup_group()?.gid;
>> +            create_options = create_options.owner(uid).group(gid);
>> +        }
>> +
>> +        let _lock = self.mutex.lock();
>> +
>> +        proxmox_sys::fs::replace_file(&chunk_path, &[], create_options, false)
>> +            .map_err(|err| format_err!("clear chunk failed for {digest_str} - {err}"))?;
>> +        Ok(())
>> +    }
>> +
>>       pub fn relative_path(&self, path: &Path) -> PathBuf {
>>           // unwrap: only `None` in unit tests
>>           assert!(self.locker.is_some());
>> diff --git a/pbs-datastore/src/local_datastore_lru_cache.rs b/pbs-datastore/src/local_datastore_lru_cache.rs
>> index 1d2e87cb9..6f950f4b3 100644
>> --- a/pbs-datastore/src/local_datastore_lru_cache.rs
>> +++ b/pbs-datastore/src/local_datastore_lru_cache.rs
>> @@ -71,17 +71,8 @@ impl LocalDatastoreLruCache {
>>       /// Fails if the chunk cannot be inserted successfully.
>>       pub fn insert(&self, digest: &[u8; 32], chunk: &DataBlob) -> Result<(), Error> {
>>           self.store.insert_chunk(chunk, digest)?;
>> -        self.cache.insert(*digest, (), |digest| {
>> -            let (path, _digest_str) = self.store.chunk_path(&digest);
>> -            // Truncate to free up space but keep the inode around, since that
>> -            // is used as marker for chunks in use by garbage collection.
>> -            if let Err(err) = nix::unistd::truncate(&path, 0) {
>> -                if err != nix::errno::Errno::ENOENT {
>> -                    return Err(Error::from(err));
>> -                }
>> -            }
>> -            Ok(())
>> -        })
>> +        self.cache
>> +            .insert(*digest, (), |digest| self.store.clear_chunk(&digest))
>>       }
>>   
>>       /// Remove a chunk from the local datastore cache.
>> @@ -104,17 +95,7 @@ impl LocalDatastoreLruCache {
>>       ) -> Result<Option<DataBlob>, Error> {
>>           if self
>>               .cache
>> -            .access(*digest, cacher, |digest| {
>> -                let (path, _digest_str) = self.store.chunk_path(&digest);
>> -                // Truncate to free up space but keep the inode around, since that
>> -                // is used as marker for chunks in use by garbage collection.
>> -                if let Err(err) = nix::unistd::truncate(&path, 0) {
>> -                    if err != nix::errno::Errno::ENOENT {
>> -                        return Err(Error::from(err));
>> -                    }
>> -                }
>> -                Ok(())
>> -            })
>> +            .access(*digest, cacher, |digest| self.store.clear_chunk(&digest))
>>               .await?
>>               .is_some()
>>           {
>> -- 
>> 2.47.3
>>
>>
>>
>> _______________________________________________
>> pbs-devel mailing list
>> pbs-devel at lists.proxmox.com
>> https://lists.proxmox.com/cgi-bin/mailman/listinfo/pbs-devel
>>
>>
>>
> 
> 
> _______________________________________________
> pbs-devel mailing list
> pbs-devel at lists.proxmox.com
> https://lists.proxmox.com/cgi-bin/mailman/listinfo/pbs-devel
> 
>