doc: update erasure code recovery description

Signed-off-by: Greg Farnum <gfarnum@redhat.com>

diff --git a/doc/rados/operations/erasure-code.rst b/doc/rados/operations/erasure-code.rst
index a94ee5a252f13b2ce43bbf1e4a51ced3376ca3b0..c49d06bb52e72016aa49ab1c73e3e055ef80f962 100644 (file)
--- a/doc/rados/operations/erasure-code.rst
+++ b/doc/rados/operations/erasure-code.rst
@@ -165,11 +165,17 @@ More information can be found in the `cache tiering
 
 Erasure coded pool recovery
 ---------------------------
-
-Erasure coded pools could not recover when there were less than min_size
-shards available. This restriction has been removed in Octopus, which means
-that as long as there are enough shards available for data to be recovered,
-we will allow recovery to proceed below min_size.
+If an erasure coded pool loses some shards, it must recover them from the others.
+This generally involves reading from the remaining shards, reconstructing the data, and
+writing it to the new peer.
+In Octopus, erasure coded pools can recover as long as there are at least *K* shards
+available. (With fewer than *K* shards, you have actually lost data!)
+
+Prior to Octopus, erasure coded pools required at least *min_size* shards to be
+available. (We generally recommend min_size be *K+2* or more to prevent loss of writes and data.)
+This conservative decision was made out of an abundance of caution when designing the new pool
+mode but also meant pools with lost OSDs but no data loss were unable to recover and go active
+without manual intervention to change the *min_size*.
 
 Glossary
 --------