Here is a tip for the webmasters (or system admins) to discover whether the nginx using proxy_cache to cache files is deleting files at the moment! There situation where you may need to know if the loaded of a static media server is caused by the deletion of the cache manager or by the read or seek operations when serving the static files. The deletion is really slow and IO intensive operation, which could greatly impact the performance and traffic of the server.
Find the process nginx’s “cache manager process” and strace it:
[root@srv ~]# ps axuf|grep nginx
root 31582 0.0 0.0 2906768 25108 ? Ss Feb15 0:01 nginx: master process /usr/sbin/nginx -c /etc/nginx/nginx.conf
nginx 16008 1.9 1.3 2941188 440224 ? S 16:39 1:33 \_ nginx: worker process
nginx 16009 1.5 1.2 2941188 398836 ? S 16:39 1:12 \_ nginx: worker process
nginx 16010 0.5 0.7 2941984 239064 ? S 16:39 0:26 \_ nginx: worker process
nginx 16011 0.7 0.9 2941984 299356 ? D 16:39 0:35 \_ nginx: worker process
nginx 16012 1.2 1.1 2941188 389540 ? D 16:39 1:01 \_ nginx: worker process
nginx 16013 2.3 1.5 2941188 487324 ? D 16:39 1:55 \_ nginx: worker process
nginx 16014 0.0 0.6 2906772 224004 ? S 16:39 0:01 \_ nginx: cache manager process
[root@srv ~]# strace -f -p 16014
strace: Process 16014 attached
gettid() = 16014
write(31, "2019/02/25 18:00:31 [info] 16014"..., 89) = 89
epoll_wait(36, [], 512, 5406) = 0
unlink("/mnt/cache/0/39/c8ccbbc06d16debb1c8d58ceb6f99390") = 0
unlink("/mnt/cache/0/78/118924d7bf70e20fa8f790c6f9e7c780") = 0
unlink("/mnt/cache/3/ce/fab074cc670e6a80114dcbc398a63ce3") = 0
unlink("/mnt/cache/5/48/0b4e162dd7be8244815721fb7d68e485") = 0
unlink("/mnt/cache/5/56/e5eb4b38c7c8d209d0aabaf79ac02565") = 0
unlink("/mnt/cache/e/c6/207b432fa77375e4eefcaf52db250c6e") = 0
unlink("/mnt/cache/4/6d/ac0db27a03dabc79d869068db1b516d4") = 0
unlink("/mnt/cache/9/e8/91625c6e60de8e5425c4135c7dfb2e89") = 0
unlink("/mnt/cache/b/3c/f3c53000cf0cb20d55d8c09df8a733cb") = 0
unlink("/mnt/cache/f/f7/6f06423cd411b45816969fe020903f7f") = 0
unlink("/mnt/cache/f/50/c9b8ab72821a6e9bcb9c8d4b790dc50f") = 0
unlink("/mnt/cache/6/1f/74b0f1fdf1ac30db6af7793dc15671f6") = 0
unlink("/mnt/cache/0/83/caf199c1b99d438f96caec71bf2ea830") = 0
unlink("/mnt/cache/4/3d/c90f8fbbba4aaf407e386641dc2203d4") = 0
unlink("/mnt/cache/4/ad/d23cf8598020141b2bcec46d2b5cbad4") = 0
unlink("/mnt/cache/d/47/05973bc310503f36c67b7c1c24c8247d") = 0
unlink("/mnt/cache/f/11/e4fcbde8533d89105ab41f22c55e211f") = 0
unlink("/mnt/cache/2/06/29066a58e4116d24266026b4ed1e3062") = 0
epoll_wait(32, [], 512, 50) = 0
unlink("/mnt/cache/4/6b/9a104ebdf70d00137a88d4584b2bb6b4") = 0
unlink("/mnt/cache/e/95/6d176447f57f21769d86a8f0b2a8b95e") = 0
unlink("/mnt/cache/b/b2/2f6f51163c65ae1fc06a913d6de1ab2b") = 0
unlink("/mnt/cache/a/24/2b058045a23b69de7a4442c9e6fce24a") = 0
unlink("/mnt/cache/7/60/00833e0b236ca8472f5be8227d645607") = 0
unlink("/mnt/cache/a/08/bf00eea300eff97dc4fffa61daaca08a") = 0
unlink("/mnt/cache/2/48/a291d8aca2b6f4f9471686eabe9b2482") = 0
unlink("/mnt/cache/0/e3/2d631adbc3bfdf8e44a51fa5453eee30") = 0
unlink("/mnt/cache/1/3b/08eef7c86c5ece9b5279b304dd86e3b1") = 0
unlink("/mnt/cache/b/a4/03213e4a8a1e8fb17ae698e54e70fa4b") = 0
unlink("/mnt/cache/b/a3/77f1b11811a9cda0ae93c498769f7a3b") = 0
unlink("/mnt/cache/4/01/1d50fac60681ae3263c8875775d20014") = 0
unlink("/mnt/cache/c/94/e71b96cbc65b248bd8e4540cbd69294c") = 0
unlink("/mnt/cache/1/59/99ec58e865b97e217835dd84f5f48591") = 0
unlink("/mnt/cache/4/b8/6a64825ce555b8f2440f051a7f7bcb84") = 0
unlink("/mnt/cache/7/51/fe2acbb895427ed8e406ce7e79d61517") = 0
.....
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You can tune the file removing from the cache with manager_files, manager_threshold and manager_sleep arguments of the proxy_cache_path.
If you came here searching information on the topic probably you should check out these articles, too: how to disable effectively the deleting (purging) files from nginx proxy_cache (nginx cache manager process) and Tune nginx proxy cache – control the cache manager how to delete cached files