深入探讨Varnish缓存命中率

也许你还在为刚才动态内容获得7336.76 reqs/s的吞吐率感到振奋，等等，理想和现实是有差距的，你要忍受现实的残酷，别忘了，我们压力测试中的动态内容都处于全缓存情况下，也就是每次请求都命中缓存，这在现实中往往是不可能的。

首先，缓存区空间大小是有限的，而我们的站点可能有大量的内容需要被缓存，而不像前边压力测试时只有一个内容。一旦缓存区被装满，那么缓存管理器便会淘汰一些它认为不再需要的缓存内容，比如通过LRU(最近最少使用算法)将使用频率较低的缓存内容淘汰出去，但是，这里判断“不常使用”的标准是不严格的，也许被淘汰的内容就是你将要访问的下一个内容，这便影响了它的命中率。

其次，缓存的过期时间也影响到它的命中率，假如有效期很短，为10秒，那么最少10秒便会有一次无法命中。

还有，有些内容可能根本没有被代理服务器缓存，比如这些内容包含了set-cookie等不可缓存的HTTP头信息，导致反向代理不会缓存它们，并且在浏览器请求它们的时候也不会去缓存区查找。这是影响命中率的一个重要因素，但往往被我们所忽略。

幸运的是，这些问题我们都可以轻松的解决，前提是，我们需要了解反向代理缓存的实时工作状态，比如Varnish便提供了一个命令行的状态监控程序varnishstat，我们打开它，便看到了当前时刻的状态，如下：

client_conn 9908723 94.05 Client connections accepted

client_drop 0 0.00 Connection dropped, no sess/wrk

client_req 16433490 155.99 Client requests received

cache_hit 8751732 83.07 Cache hits

cache_hitpass 42592 0.40 Cache hits for pass

cache_miss 7573389 71.89 Cache misses

backend_conn 3889845 36.92 Backend conn. success

backend_unhealthy 220 0.00 Backend conn. not attempted

backend_busy 0 0.00 Backend conn. too many

backend_fail 4536 0.04 Backend conn. failures

backend_reuse 3780212 35.88 Backend conn. reuses

backend_toolate 3866687 36.70 Backend conn. was closed

backend_recycle 7646677 72.58 Backend conn. recycles

backend_unused 0 0.00 Backend conn. unused

fetch_head 57 0.00 Fetch head

fetch_length 155097 1.47 Fetch with Length

fetch_chunked 7508522 71.27 Fetch chunked

fetch_eof 0 0.00 Fetch EOF

fetch_bad 0 0.00 Fetch had bad headers

fetch_close 3982 0.04 Fetch wanted close

fetch_oldhttp 0 0.00 Fetch pre HTTP/1.1 closed

fetch_zero 0 0.00 Fetch zero len

fetch_failed 0 0.00 Fetch failed

n_sess_mem 1033 . N struct sess_mem

n_sess 633 . N struct sess

n_object 1016443 . N struct object

n_vampireobject 0 . N unresurrected objects

n_objectcore 1017564 . N struct objectcore

n_objecthead 982903 . N struct objecthead

n_smf 2647421 . N struct smf

n_smf_frag 622470 . N small free smf

n_smf_large 3 . N large free smf

n_vbe_conn 12 . N struct vbe_conn

n_wrk 8000 . N worker threads

n_wrk_create 8000 0.08 N worker threads created

n_wrk_failed 0 0.00 N worker threads not created

n_wrk_max 11021 0.10 N worker threads limited

n_wrk_queue 0 0.00 N queued work requests

n_wrk_overflow 2441 0.02 N overflowed work requests

n_wrk_drop 0 0.00 N dropped work requests

n_backend 4 . N backends

n_expired 6344546 . N expired objects

n_lru_nuked 183957 . N LRU nuked objects

n_lru_saved 0 . N LRU saved objects

n_lru_moved 3692170 . N LRU moved objects

n_deathrow 0 . N objects on deathrow

losthdr 84 0.00 HTTP header overflows

n_objsendfile 0 0.00 Objects sent with sendfile

n_objwrite 15466812 146.81 Objects sent with write

n_objoverflow 0 0.00 Objects overflowing workspace

s_sess 9906155 94.03 Total Sessions

s_req 16433490 155.99 Total Requests

s_pipe 37 0.00 Total pipe

s_pass 108252 1.03 Total pass

s_fetch 7667658 72.78 Total fetch

s_hdrbytes 7187255662 68221.35 Total header bytes

s_bodybytes 111592032839 1059230.32 Total body bytes

sess_closed 1905544 18.09 Session Closed

sess_pipeline 0 0.00 Session Pipeline

sess_readahead 0 0.00 Session Read Ahead

sess_linger 15277717 145.02 Session Linger

sess_herd 13547370 128.59 Session herd

shm_records 1028855796 9765.89 SHM records

shm_writes 77957008 739.97 SHM writes

shm_flushes 131005 1.24 SHM flushes due to overflow

shm_cont 144281 1.37 SHM MTX contention

shm_cycles 427 0.00 SHM cycles through buffer

sm_nreq 15306717 145.29 allocator requests

sm_nobj 2024948 . outstanding allocations

sm_balloc 13595295744 . bytes allocated

sm_bfree 40091795456 . bytes free

sma_nreq 0 0.00 SMA allocator requests

sma_nobj 0 . SMA outstanding allocations

sma_nbytes 0 . SMA outstanding bytes

sma_balloc 0 . SMA bytes allocated

sma_bfree 0 . SMA bytes free

sms_nreq 14062 0.13 SMS allocator requests

sms_nobj 0 . SMS outstanding allocations

sms_nbytes 487 . SMS outstanding bytes

sms_balloc 6844837 . SMS bytes allocated

sms_bfree 6846298 . SMS bytes freed

backend_req 7668789 72.79 Backend requests made

n_vcl 1 0.00 N vcl total

n_vcl_avail 1 0.00 N vcl available

n_vcl_discard 0 0.00 N vcl discarded