Optionally write stub entries to the chunk cache from ingesters

docs/arguments.md

@@ -116,6 +116,11 @@ The ingester query API was improved over time, but defaults to the old behaviour
 
    When `push` requests arrive, pre-allocate this many slots to decode them. Tune this setting to reduce memory allocations and garbage. The optimum value will depend on how many labels are sent with your timeseries samples.
 
+- `-store.chunk-cache-stubs`
+
+   Where you don't want to cache every chunk written by ingesters, but you do want to take advantage of chunk write deduplication, this option will make ingesters write a placeholder to the cache for each chunk.
+   Make sure you configure ingesters with a different cache to queriers, which need the whole value.
+
 ## Ingester, Distributor & Querier limits.
 
 Cortex implements various limits on the requests it can process, in order to prevent a single tenant overwhelming the cluster.  There are various default global limits which apply to all tenants which can be set on the command line.  These limits can also be overridden on a per-tenant basis, using a configuration file.  Specify the filename for the override configuration file using the `-limits.per-user-override-config=<filename>` flag.  The override file will be re-read every 10 seconds by default - this can also be controlled using the `-limits.per-user-override-period=10s` flag.

pkg/chunk/cache/cache_test.go

@@ -96,7 +96,7 @@ func testCacheMultiple(t *testing.T, cache cache.Cache, keys []string, chunks []
 func testChunkFetcher(t *testing.T, c cache.Cache, keys []string, chunks []chunk.Chunk) {
 	fetcher, err := chunk.NewChunkFetcher(cache.Config{
 		Cache: c,
-	}, nil)
+	}, false, nil)
 	require.NoError(t, err)
 	defer fetcher.Stop()
 

pkg/chunk/chunk_store.go

@@ -64,11 +64,15 @@ type StoreConfig struct {
 
 	// Limits query start time to be greater than now() - MaxLookBackPeriod, if set.
 	MaxLookBackPeriod time.Duration `yaml:"max_look_back_period"`
+
+	// Not visible in yaml because the setting shouldn't be common between ingesters and queriers
+	chunkCacheStubs bool // don't write the full chunk to cache, just a stub entry
 }
 
 // RegisterFlags adds the flags required to config this to the given FlagSet
 func (cfg *StoreConfig) RegisterFlags(f *flag.FlagSet) {
 	cfg.ChunkCacheConfig.RegisterFlagsWithPrefix("", "Cache config for chunks. ", f)
+	f.BoolVar(&cfg.chunkCacheStubs, "store.chunk-cache-stubs", false, "If true, don't write the full chunk to cache, just a stub entry.")
 	cfg.WriteDedupeCacheConfig.RegisterFlagsWithPrefix("store.index-cache-write.", "Cache config for index entry writing. ", f)
 
 	f.DurationVar(&cfg.MinChunkAge, "store.min-chunk-age", 0, "Minimum time between chunk update and being saved to the store.")
@@ -92,7 +96,7 @@ type store struct {
 }
 
 func newStore(cfg StoreConfig, schema Schema, index IndexClient, chunks ObjectClient, limits *validation.Overrides) (Store, error) {
-	fetcher, err := NewChunkFetcher(cfg.ChunkCacheConfig, chunks)
+	fetcher, err := NewChunkFetcher(cfg.ChunkCacheConfig, cfg.chunkCacheStubs, chunks)
 	if err != nil {
 		return nil, err
 	}

pkg/chunk/chunk_store_utils.go

@@ -54,8 +54,9 @@ outer:
 // and writing back any misses to the cache.  Also responsible for decoding
 // chunks from the cache, in parallel.
 type Fetcher struct {
-	storage ObjectClient
-	cache   cache.Cache
+	storage    ObjectClient
+	cache      cache.Cache
+	cacheStubs bool
 
 	wait           sync.WaitGroup
 	decodeRequests chan decodeRequest
@@ -72,7 +73,7 @@ type decodeResponse struct {
 }
 
 // NewChunkFetcher makes a new ChunkFetcher.
-func NewChunkFetcher(cfg cache.Config, storage ObjectClient) (*Fetcher, error) {
+func NewChunkFetcher(cfg cache.Config, cacheStubs bool, storage ObjectClient) (*Fetcher, error) {
 	cache, err := cache.New(cfg)
 	if err != nil {
 		return nil, err
@@ -81,6 +82,7 @@ func NewChunkFetcher(cfg cache.Config, storage ObjectClient) (*Fetcher, error) {
 	c := &Fetcher{
 		storage:        storage,
 		cache:          cache,
+		cacheStubs:     cacheStubs,
 		decodeRequests: make(chan decodeRequest),
 	}
 
@@ -149,10 +151,14 @@ func (c *Fetcher) writeBackCache(ctx context.Context, chunks []Chunk) error {
 	keys := make([]string, 0, len(chunks))
 	bufs := make([][]byte, 0, len(chunks))
 	for i := range chunks {
-		encoded, err := chunks[i].Encoded()
-		// TODO don't fail, just log and conitnue?
-		if err != nil {
-			return err
+		var encoded []byte
+		var err error
+		if !c.cacheStubs {
+			encoded, err = chunks[i].Encoded()
+			// TODO don't fail, just log and conitnue?
+			if err != nil {
+				return err
+			}
 		}
 
 		keys = append(keys, chunks[i].ExternalKey())

pkg/chunk/series_store.go

@@ -68,7 +68,7 @@ type seriesStore struct {
 }
 
 func newSeriesStore(cfg StoreConfig, schema Schema, index IndexClient, chunks ObjectClient, limits *validation.Overrides) (Store, error) {
-	fetcher, err := NewChunkFetcher(cfg.ChunkCacheConfig, chunks)
+	fetcher, err := NewChunkFetcher(cfg.ChunkCacheConfig, cfg.chunkCacheStubs, chunks)
 	if err != nil {
 		return nil, err
 	}