Description of problem:

Seen in squid's cache.log: aio_queue_request: WARNING - Queue congestion

Based on this thread, it appears that the number of threads available to squid
needs to be increased:

http://ftp.pop-mg.rnp.br/squid/mail-archive/squid-users/200111/1228.html

The current squid settings use these defaults:

#ifdef ASYNC_IO_THREADS
#define NUMTHREADS ASYNC_IO_THREADS
#else
#define NUMTHREADS (Config.cacheSwap.n_configured*16)
#endif

which defines 16 threads per cache directory configured in squid.conf.
(reference parse_cachedir() in src/cache_cf.c)

I think that changing the configure line for squid to have
"--with-aio-threads=32" would be a potential starting point.

Either that, or patching the above portion in src/fs/aufs/store_asyncufs.h to be
*32 per cache directory, so that sites already using multiple cache directories
will not be suprised by the overall number of threads decreasing if they have
more than 2 directories setup.