proxy cache make system load and I/O high

eagle sbc sbc19861004 at
Wed Dec 22 16:21:14 MSK 2010

On Wed, Dec 22, 2010 at 9:09 PM, eagle sbc <sbc19861004 at> wrote:

> Hi, all:
> This really exhaust me.
> I had an nginx server with "proxy cache" on, the num of cache files is
> almost 500K, and the files size are all near 50K.  But now with just not up
> to 200 connections, the "load average" is almost to "3", and the I/O of the
> cache disk  is "1000KB/s", and with lots of connections "TIME_OUT".  I add
> the $upstream_cache_status variable to the log, and in the result, 70% are
> not using proxy cache, 20% are missing and only 10% are hit.
> But I had a same server with "proxy store" on, and the num of cache files
> is also 500K. But on this server, even with 1000 connections, the "load
> average" is never up to "0.3", and the I/O for read is just up to "200KB/s",
> and all connections are in a "ESTABLISHED" or "FIT_WAIT" status.
> My question is :
>    1. Why with so less connections, the I/O is still so high?
>    2. What other operation does "proxy cache" do opposite to "proxy
>    store"? Aren't they all look up the file, and: if found, read file from
>    disk; if not, proxy to the upstream server? While in fact they are so
>    different on the "load average".
I also made a test that using memory to make a tmpfs, and locate the cache
all in memory. And the 'load average' performance is then down to '0.3'. But
my server can only spare 1.5G memory for the tmpfs, and even with
'max_size=1G' configured, nginx doesn't recycle the files, but soon the
filesystem is filling up(may be this is not correct, I seen some memory
space freed, but if I configured the max_size=100G for the disk filesystem.
when the cache using up to 1G, their also some disk space freed. So I think
maybe may other configuration caused that recycle). That's wired. Is that my
new writes too frequent than the recycle?
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