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Recover from Table Segment Pre-Index Full


Hash-based Table Segments are used internally to store metadata in Pravega. One key aspect of the operation of Hash-based Table Segments is that it indexes keys and values in an offline fashion: first, PUT and/or REMOVE operations are written to Tier-1, and then, the Attribute Index is generated and updated in LTS as soon as data for that Table Segment is processed and moved to LTS. For more details, please see PDP 48.

The offline approach to build the Attribute Index adopted in Hash-based Table Segments leads to having a backlog of PUT and/or REMOVE operations that are stored in Tier-1, but not yet indexed and stored in LTS. If LTS is unavailable for a long period of time and one of such Hash-based Table Segments continues to receive load, there is a risk of having too much data being accumulated in Tier-1 only. This may be especially problematic as keys and values stored in Tier-1 are also cached in memory to guarantee consistency, in a process called Table Segment "pre-index" or "tail-caching". Due to this, there are a few parameters that limit the amount of non-indexed (i.e., written in Tier-1 but not yet to LTS) data that Hash-based Table Segments can tolerate: - tables.preindex.bytes.max (defaults to 128MB): The maximum un-indexed data length for which a Hash-based Table Segment can perform tail-caching. - tables.recovery.timeout.millis (defaults to 60 seconds): The maximum amount of time to wait for a Table Segment Recovery. - tables.unindexed.bytes.max (defaults to 128MB): The maximum allowed un-indexed data length for a Segment. Any updates that occur while the amount of un-indexed data exceeds this threshold will be blocked until the amount of un-indexed data is reduced. - tables.systemcritical.unindexed.bytes.max (defaults to 256MB): Same as the previous parameter, but for Metadata Table Segments (i.e., container_metadata and storage_metadata on each Segment Container).

In rare cases in which LTS is super slow or unavailable for hours, but there is some activity happening in a Hash-based Table Segment, we can find errors in the logs such as:

2022-08-08 13:07:57,194 1192230051 [core-19] INFO i.p.s.s.tables.ContainerKeyIndex - KeyIndex[27]: Table Segment 14 cannot perform tail-caching because tail index too long (150542984).
2022-08-08 13:07:57,194 1192230051 [core-19] INFO i.p.s.s.tables.ContainerKeyIndex - KeyIndex[27]:: System-critical TableSegment 3 is blocked due to reaching max unindexed size.

Messages like the ones above indicate that Table Segments may not be able to recover, to write new entries, or both. If the impacted Hash-based Table Segments are critical for the operation of Pravega, it may leave the impacted Segment Container unable for processing new operations. Once this situation is reached, recovering the Segment Container requires manual intervention.

Repair Procedure

The repair procedure is relatively simple and works as follows:

  1. Make sure that LTS is available and works properly: The root cause of the problem is related to LTS not being able to store data fast enough, or just being unavailable for a very long period of time. Any attempt to recover Pravega first requires to be sure that LTS is working fine.

  2. Temporarily update Table Segment un-indexed data parameters: Once LTS is back to working state, we need to increase the values in the aforementioned parameters. Concretely, the following errors related to the configuration parameters: i) ...tail index too long. requires increasing tables.preindex.bytes.max, ii) ...recovery timed out. requires increasing tables.recovery.timeout.millis, iii) ...blocked due to reaching max unindexed size requires increasing tables.unindexed.bytes.max or tables.systemcritical.unindexed.bytes.max, depending on whether the Table Segment is a metadata one (i.e., container_metadata and storage_metadata on each Segment Container) or not. The values to be set should be enough to get through the blocker issue. For instance, if one Table Segment shows a log like cannot perform tail-caching because tail index too long (150542984), it is reasonable to configure tables.preindex.bytes.max to 256MBs. However, setting an excessively high value may be problematic in the case that LTS is still not working properly, as data may continue getting accumulated up to the configured value. If multiple error messages are found related to Table Segments, you can update all these parameters to higher values. Also, note that the "pre-index" or "tail-caching" process involves storing data in cache. For this reason, in some cases, it may be necessary to also temporarily increase cache size (i.e.,pravegaservice.cache.size.max) and Direct Memory as well (see Segment Store Cache Size and Memory Settings).

  3. Inspect metrics and confirm that un-indexed data is actually being consumed: Once the parameters have been increased, we need to be sure that the un-indexed data for Table Segments is being processed. To this end, a recommended procedure is to keep an eye to the segmentstore.tablesegment.used_credits metric. Once you confirm that this metric decreases for the relevant Table Segments, it is a clear indication that Pravega is moving data to LTS, including the Table Segment operations to be indexed. An example of analyzing this metric can be found in this issue.

  4. Revert changes in Table Segment un-indexed data parameters: Once all Table Segments have consumed the outstanding un-indexed data, it is time to revert the configuration changes made to recover the cluster. Otherwise, if we find again a long-lasting problem with LTS, some Table Segments may accumulate much more data this time, as the bounds for un-indexed data have been increased (the more accumulated data, the more complex it is to recover the cluster).