Add APM Server known issue for TBS (#4862)

* Add APM Server known issue for TBS

Signed-off-by: Marc Lopez Rubio <[email protected]>

* Apply suggestions from code review

Co-authored-by: Colleen McGinnis <[email protected]>

---------

Signed-off-by: Marc Lopez Rubio <[email protected]>
Co-authored-by: Brandon Morelli <[email protected]>
Co-authored-by: Colleen McGinnis <[email protected]>

Author: 3 people

docs/en/observability/apm/known-issues.asciidoc

@@ -21,6 +21,17 @@ _Versions: XX.XX.XX, YY.YY.YY, ZZ.ZZ.ZZ_
 // If applicable, link to fix
 ////
 
+[discrete]
+== Tail Sampling may not compact / expired TTLs as quickly as desired, causing increased storage usage.
+
+_Elastic Stack versions: All 8.x versions_
+
+There are some issues with the tail sampling implementation in all 8.x versions that may prevent buffered traces from being compacted or expired as quickly as expected. This can lead to increased storage usage for longer than the default 30m TTL.
+
+This may manifest in two ways, increased value log (vlog) file size and increased SST (LSM) file size. LSM growth and late compaction is particularly troublesome given how the underlying K/V database performs compactions on its layers. There is noticeable LSM growth for use-cases where traces are under 1KB in size, since they are written to the LSM layer directly.
+
+This issue is fixed in 9.0.0, due to a re-implementation of how the underlying tail sampling databases are used. The new implementation uses a more efficient partitioning scheme, allowing more efficient expiration of traces.
+
 [discrete]
 == APM Server v8.6.x and prior with Elasticsearch v8.15.x and later has broken APM UI