Re: [pbs-devel] [PATCH proxmox-backup] GC: s3: fix local marker cleanup for unreferenced, s3 only chunks

[Christian Ebner <c.ebner@proxmox.com>]

On 11/24/25 9:13 AM, Fabian Grünbichler wrote:
> On November 24, 2025 8:35 am, Christian Ebner wrote:
>> Thanks for taking a look at this already!
>>
>> On 11/22/25 3:55 PM, Thomas Lamprecht wrote:
>>> Am 22.11.25 um 11:41 schrieb Christian Ebner:
>>>> If a chunk object is located on the s3 object store only, not being
>>>> referenced by any index file and having no local marker file it is
>>>> marked for cleanup by pretending an atime equal to the unix epoch.
>>>>
>>>> While this will mark the chunk for deletion from the backend and
>>>> include it in the delete list for the next s3 delete objects call, it
>>>> also will lead to the chunk marker and LRU cache entry being tried to
>>>> clean up locally, which however fails since there is no marker to be
>>>> cleaned up.
>>>>
>>>> In order to treat this edge case with the same cleanup logic, simply
>>>> insert the marker file if not present, for it to get correctly
>>>> cleaned up as expected afterwards. This should not happen under
>>>> normal datastore operation anyways, most likely to appear after
>>>> re-creation of the datastore from existing bucket contents containing
>>>> such unreferenced chunks.
>>>>
>>>> Fixes: https://forum.proxmox.com/threads/176567/
>>>> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
>>>> ---
>>>>    pbs-datastore/src/datastore.rs | 9 +++++----
>>>>    1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
>>>> index 65299cca9..a24392d9f 100644
>>>> --- a/pbs-datastore/src/datastore.rs
>>>> +++ b/pbs-datastore/src/datastore.rs
>>>> @@ -1711,11 +1711,12 @@ impl DataStore {
>>>>                        let atime = match std::fs::metadata(&chunk_path) {
>>>>                            Ok(stat) => stat.accessed()?,
>>>>                            Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
>>>> +                            unsafe {
>>>> +                                // chunk store lock held
>>>> +                                // insert marke unconditionally, cleaned up again below if required
>>>> +                                self.inner.chunk_store.replace_chunk_with_marker(&digest)?;
>>>> +                            }
>>>>                                if self.inner.chunk_store.clear_chunk_expected_mark(&digest)? {
>>>> -                                unsafe {
>>>> -                                    // chunk store lock held
>>>> -                                    self.inner.chunk_store.replace_chunk_with_marker(&digest)?;
>>>> -                                }
>>>>                                    SystemTime::now()
>>>
>>> Why not drop that whole branch instead, it does not really makes sense IIUC.
>>
>> No, this branch is needed. This is required for avoiding API calls to
>> the s3 backend in case the chunk is referenced by an index file as
>> detected during phase 1, but the local marker file is not present. In
>> that case we do not want to directly check the existence on the backend
>> (which we need to make sure to not mark a chunk which is however not
>> present on the backend), but defer that check to phase 2, where we do
>> the listing of all chunks anyways. This is done by flagging that chunk
>> via the <digest>.using file.
>>
>> Here, if the chunk is encountered during s3 object store listing, but
>> the local file is missing, we check and clear the chunk expected marker,
>> which if present tells us the chunk still needs to be used. If not it is
>> safe to clear it from the backend.
>>
>>>
>>> And `replace_chunk_with_marker` replaces the chunk file directly (no extension) whereas
>>> `clear_chunk_expected_mark` checks the chunk.using file, so does your reordering even
>>> change anything, or is there a bug in `replace_chunk_with_marker`?
>>
>> `replace_chunk_with_marker` replaces a full chunk file with an empty
>> marker, but also creates the empty marker if the original file is not
>> present, so in this particular case it is actually used to create the
>> marker, not to evict chunks from local datastore cache as under normal
>> operation. I can send a patch to rename that method to make that clear.
>>
>>>
>>> And independent of that, would it be better (more performant and less confusing) if
>>> we ignore the "not present in LRU or no marker" in that edge case rather than creating
>>> a file (doing more IO) just to delete that then again?
>>
>> I can do that as well of course by checking a flag in the remove
>> callback. I opted for not doing that however since above is a very
>> unlikely case to happen, as the s3 backend and local datastore cache
>> should be in sync most of the time.
>> Adding that check would be performed for each chunk being removed, this
>> only once if the chunk is still present on the backend, but not on the
>> local datastore cache.
>>
>> The additional IO is therefore justfied IMO.
>>
>> I could of course also go the route of just setting a boolean flag and
>> checking that in the callback?
> 
> we basically already have such a boolean marker - we set `atime` to 0 in
> this case (and only this case), and we could just ignore the removal
> errors then? possibly limited to just ignoring ENOTFOUND?

That's a good idea! So I will perform the additional checks based on that.



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