From 0dea6d8379165c2f38125f8bb308733f1e4cb4ec Mon Sep 17 00:00:00 2001
From: Matthew Ahrens <mahrens@delphix.com>
Date: Tue, 26 Jul 2022 11:05:58 -0700
Subject: [PATCH] fix import/discovery panic (#523)

When starting the agent and discovering zettacaches, if there's an old
zettacache (with an older on-disk format) present, the agent crashes.
We should instead ignore this old cache.

Bonus cleanup: document recommended relationship between tunables.
---
 cmd/zfs_object_agent/zettacache/src/open.rs     | 17 ++++++++---------
 .../zettacache/src/slab_allocator.rs            |  1 +
 .../zettacache/src/zettacache/mod.rs            |  3 ++-
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/cmd/zfs_object_agent/zettacache/src/open.rs b/cmd/zfs_object_agent/zettacache/src/open.rs
index d738d28960d7..8a2791d9a70c 100644
--- a/cmd/zfs_object_agent/zettacache/src/open.rs
+++ b/cmd/zfs_object_agent/zettacache/src/open.rs
@@ -183,15 +183,14 @@ fn is_valid_cache(
 
     match primary {
         Some(disk) => {
-            let disks_from_primary = disk
-                .superblock
-                .primary
-                .as_ref()
-                .unwrap()
-                .disks
-                .iter()
-                .map(|(k, v)| (*k, v.guid))
-                .collect::<HashMap<_, _>>();
+            let disks_from_primary = match &disk.superblock.primary {
+                Some(primary) => primary
+                    .disks
+                    .iter()
+                    .map(|(k, v)| (*k, v.guid))
+                    .collect::<HashMap<_, _>>(),
+                None => return false,
+            };
 
             // Only retain disks with IDs that are part of the primary block
             disks.retain(
diff --git a/cmd/zfs_object_agent/zettacache/src/slab_allocator.rs b/cmd/zfs_object_agent/zettacache/src/slab_allocator.rs
index b895720cf95c..cfb33d07734c 100644
--- a/cmd/zfs_object_agent/zettacache/src/slab_allocator.rs
+++ b/cmd/zfs_object_agent/zettacache/src/slab_allocator.rs
@@ -53,6 +53,7 @@ tunable! {
 
     // Try to keep this amount of slabs available for normal (i.e. BlockAllocator) use.  Ideally
     // this will be enough space to absorb all the writes that can occur between index merges.
+    // This should be less than TARGET_FREE_BLOCKS_PCT.
     static ref TARGET_AVAILABLE_SLABS_PCT: Percent = Percent::new(2.0);
 }
 
diff --git a/cmd/zfs_object_agent/zettacache/src/zettacache/mod.rs b/cmd/zfs_object_agent/zettacache/src/zettacache/mod.rs
index f09994227a42..f780fc100ce1 100644
--- a/cmd/zfs_object_agent/zettacache/src/zettacache/mod.rs
+++ b/cmd/zfs_object_agent/zettacache/src/zettacache/mod.rs
@@ -113,7 +113,8 @@ tunable! {
     // keep at least 5% of the cache "free".  We need to have slop for the rebalance code to be
     // able to consolidate slabs (to create empty slabs) to accomodate block size changes in the
     // workload.  This target includes both free blocks within the BlockAllocator, and free slabs
-    // within the SlabAllocator which are available to the BlockAllocator.
+    // within the SlabAllocator which are available to the BlockAllocator.  This should be more
+    // than TARGET_AVAILABLE_SLABS_PCT.
     static ref TARGET_FREE_BLOCKS_PCT: Percent = Percent::new(5.0);
 
     // Keep the total footprint for the pending changes and index cache data at about 12% of