Magento Performance: Debugging Varnish Cache Fragmentation

Problem

A common production issue in Magento stores is higher server load and increased TTFB, even though Magento Varnish Cache is enabled.
In many cases, the root cause is low magento Varnish cache hit ratio, which means more requests are reaching Magento instead of being served from cache.
Cache inefficiencies can come from:

Cache misses
Aggressive pass rules
Cookie behaviour
Cache fragmentation

Diagnostics

Start by collecting Varnish runtime statistics.

varnishstat
Example snapshot:
MAIN.client_req 1200000
MAIN.cache_hit 900000
MAIN.cache_miss 120000
MAIN.cache_hitpass 80000
MAIN.backend_req 200000
MAIN.s_pass 75000
MAIN.n_lru_nuked 45000

Cache Effectiveness

Cache Hit

MAIN.cache_hit 900000
Requests served directly from magento Varnish cache.

Cache Miss

MAIN.cache_miss 120000
Requests that were not present in cache and fetched from backend.

Cache Hit Ratio

cache_hit / (cache_hit + cache_miss)
Indicates how efficiently Varnish serves cacheable traffic.

Cache Hitpass

MAIN.cache_hitpass 80000
Requests intentionally bypassing cache due to pass rules or uncacheable responses.
If this number is high, review:

Pass rules
Cookies
Headers

Backend Load Indicators

Backend Requests

MAIN.backend_req 200000
Total number of requests reaching Magento.
Higher backend requests usually correlate with higher TTFB.

Cache Memory Pressure

LRU Nuked Objects

MAIN.n_lru_nuked 45000
Objects evicted early due to memory pressure, often caused by multiple cache variants of the same page.

Total Client Requests

MAIN.client_req 1200000
Total requests received by Varnish.
Used as the base metric when evaluating cache efficiency.

Requests Passed to Backend

MAIN.s_pass 75000
Requests forced to bypass cache due to VCL pass rules.
Large values may indicate overly aggressive pass conditions.

Takeaway

When tuning Magento Varnish Cache, avoid large configuration changes at once.
Instead:

Make incremental tweaks
Monitor Varnish metrics after each change
Observe impact on hit ratio and backend requests

Also remember:
When a Magento Varnish Cache is first warmed or after configuration changes, hit ratio will initially be low. It gradually improves as cache objects populate.
The goal is not a perfect hit ratio, but a stable system where most cacheable traffic is served from Varnish and backend load remains controlled.