# Nginx Caching strategy **URL:** \ **Category:** NGINX\ **Created:** [October 4, 2010, 9:14pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987 "2010-10-04T21:14:08Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 4, 2010, 9:14pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/1 "2010-10-04T21:14:08Z")
All - Is there a high level tutorial on how nginx does caching ? Does it use mmap to map the cache directory/file into memory and share it with all nginx worker processes (and use sendfile to send it out to the socket) ? Or does it use directIO to open the file and use regular read/write syscalls ? This is for Linux debian platform. Thanks, Posted at Nginx Forum: > **[Nginx Caching strategy](https://forum.nginx.org/read.php?2,137051,137051#msg-137051)** > > All - Is there a high level tutorial on how nginx does caching ? Does it use mmap to map the cache directory/file into memory and share it with all nginx worker processes (and use sendfile to send it out to the socket) ? Or does it use directIO to... ---
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 4, 2010, 9:56pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/2 "2010-10-04T21:56:16Z")
Also a related question - If AIO is used, does Nginx maintain two levels of cache (one in-memory and one on the disk) ? Does the cache manager look at only disk caches for replacement ? Posted at Nginx Forum: > **[Re: Nginx Caching strategy](https://forum.nginx.org/read.php?2,137051,137064#msg-137064)** > > All - Is there a high level tutorial on how nginx does caching ? Does it use mmap to map the cache directory/file into memory and share it with all nginx worker processes (and use sendfile to send it out to the socket) ? Or does it use directIO to... ---
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 5, 2010, 1:32pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/3 "2010-10-05T13:32:37Z")
Hello! On Mon, Oct 04, 2010 at 03:12:57PM -0400, cachenewbie wrote: > All - Is there a high level tutorial on how nginx does caching ? Does Some information is available here (in Russian): > **[Модуль ngx\_http\_proxy\_module](http://nginx.org/ru/docs/http/ngx_http_proxy_module.html)** Translation is available here (looks like correct one, but I haven’t really checked): > **[GitHub - nginxinc/nginx-wiki: ARCHIVED -- Source for the now archived NGINX Wiki...](https://github.com/nginxinc/nginx-wiki)** > > ARCHIVED -- Source for the now archived NGINX Wiki section of https://www.nginx.com > it use mmap to map the cache directory/file into memory and share it > with all nginx worker processes (and use sendfile to send it out to the > socket) ? Or does it use directIO to open the file and use regular > read/write syscalls ? This is for Linux debian platform. nginx just saves responses to disk under the distinct names (derived from cache key), with headers at start. When reading from cache, nginx uses regular read() (or aio if available and switched on) to read headers and normal file sending mecachnism to send body. “Normal file sending mechanism” implies sendfile, aio, directio and so on - if available and configured. Some metainformation about cache contents (to count cache accesses, remove old files and so on) are stored in shared memory. Maxim D. ---
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 13, 2010, 4:19pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/4 "2010-10-13T16:19:10Z")
Thanks Maxim, Guzman. I think storing responses directly in disk makes it easy to store and retrieve from different process contexts. However, if the storage space is huge (in TB) and if the disk storage media is SSD, write avoidance is something that should be considered. In this case, small files can be stored in memory cache to speed up access to frequent webcontent and cachemanager can try to store big blocks to disks when evicting content from memory cache. Having memory cache and disk cache separated will make more sense in such a case. I was trying to figure out if Nginx supported that concept. Posted at Nginx Forum: > **[Re: Nginx Caching strategy](https://forum.nginx.org/read.php?2,137051,140088#msg-140088)** > > All - Is there a high level tutorial on how nginx does caching ? Does it use mmap to map the cache directory/file into memory and share it with all nginx worker processes (and use sendfile to send it out to the socket) ? Or does it use directIO to... ---
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 8, 2010, 2:09pm UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/5 "2010-10-08T14:09:49Z")
Hi! I’m not a nginx developer and I’ve not even seen the source yet, however I’ve nginx running on cache and I can reply by experience one of your questions. As long as you have free memory and your os (mine is debian too) correctly uses that memory to cache, you won’t see a single IO Read into the cache files by nginx. You will see the first io read to the cached files whenever you have no more free memory to cache. Please share later your experience 🙂 In my case in case it helps someone else, I had serious issues with IO but didn’t blame nginx but my current virtualization setup: XenServer from Citrix over two SATA disks using software raid-1, using lvm for partitions and having on the same host other vm’s with the backends, mysql database, etc. My IO was killing me when reaching a few hundreds requests per second, leaving sometimes most of workers locked on IO wait. After tunning and tunning nginx decided it wasn’t the one to blame and fixed it other way: added SSD disk (software RAID-1 and LVM too) to the host and mounted a partition on the nginx debian machine exclusively for nginx cache, temp folder and logs. Since then i’ve not been able to throw enough load to have a IO problem again :-). My current iostats on the production server: avg-cpu: %user %nice %system %iowait %steal %idle 0.10 0.03 0.31 0.39 0.02 99.15 Cheers Guzman Posted at Nginx Forum: > **[Re: Nginx Caching strategy](https://forum.nginx.org/read.php?2,137051,138455#msg-138455)** > > All - Is there a high level tutorial on how nginx does caching ? Does it use mmap to map the cache directory/file into memory and share it with all nginx worker processes (and use sendfile to send it out to the socket) ? Or does it use directIO to... ---
**Author:** ![cachenewbie](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/f07891/32.png) [@cachenewbie](https://www.ruby-forum.com/u/cachenewbie)\ **Post date:** [October 14, 2010, 3:51am UTC](https://www.ruby-forum.com/t/nginx-caching-strategy/195987/6 "2010-10-14T03:51:29Z")
On Wed, Oct 13, 2010 at 9:17 AM, cachenewbie [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I think storing responses directly in disk makes it easy to store and > retrieve from different process contexts. However, if the storage space > is huge (in TB) and if the disk storage media is SSD, write avoidance is > something that should be considered. In this case, small files can be > stored in memory cache to speed up access to frequent webcontent and > cachemanager can try to store big blocks to disks when evicting content > from memory cache. Having memory cache and disk cache separated will > make more sense in such a case. I was trying to figure out if Nginx > supported that concept. This is something the OS and filesystem layers can and should handle. Coding such behavior into the application layer is a bad idea for innumerable reasons (not the least of which is potability of the application). Good SSD controllers also do memory caching, block remapping, wear-leveling, and small-write coalescing internally, allowing them to be used with traditional filesystems. In short: buy good SSDs (a recent Intel or Sandforce-based drive is good) and don’t worry about it. ## Do you have any real-world data that shows that small file writes in a proxy caching scenario is a actually problem worth worrying about? As Knuth once said, “premature optimization is the root of all evil.” RPM