Caching New Added 10 September 2026

Memcached: SERVER_ERROR object too large for cache

The item exceeded the maximum item size, one megabyte by default, so it was refused rather than stored. Because most clients treat a failed set as a cache miss, the symptom is usually a page that is mysteriously slow rather than an error: every request recomputes the same oversized value.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# What the running server allows, and how often this is happening
echo "stats settings" | nc localhost 11211 | grep item_size_max
echo "stats" | nc localhost 11211 | grep -E "cmd_set|total_items"

# Raise the limit if the value genuinely has to be that big
memcached -I 4m -m 2048

# Better: store less. Compress, or split the value
# Most clients compress above a threshold: check yours is enabled
# php: memcached.compression_threshold, pylibmc: behaviors={"compression": True}

# Cache the identifiers and fetch the rows, rather than caching whole pages
# get user:42:order_ids  ->  multi get order:*

# Log the failure instead of silently missing
if (!$mc->set($key, $value)) { error_log($mc->getResultMessage()); }

How to diagnose Caching errors

Cache problems are rarely reported as cache problems. They arrive as a traffic spike that takes down the origin (a stampede after a mass eviction), users seeing old content after a deploy, or an inexplicable 403 from the CDN. The unifying diagnostic is to look at the cache status header on a real response before theorising about anything else.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Read the cache status header on a live request: curl -sI https://example.com | grep -i -E 'cache|age|x-cache|cf-cache-status'. HIT, MISS and BYPASS each point at a different root cause.
Check the Age header against your intended TTL. An Age larger than max-age means something is serving stale content deliberately (stale-while-revalidate), often the desired behaviour, occasionally the bug.
For stampedes, add request coalescing or a short randomised TTL jitter rather than a longer TTL. Identical expiry times across many keys are what create the thundering herd.
Use content-hashed filenames for static assets and no-cache for HTML. Almost every "users see the old version" incident traces back to a long TTL on an HTML document.
For CDN 403s, distinguish a CDN-generated response from an origin one by checking whether CDN-specific headers are present. An origin 403 has a completely different fix.

Tools worth reaching for

curl -sI
varnishlog
CDN edge logs
Cache-Control validators

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

github.com

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