The caching you are referring to is not in userspace; it's the Linux kernel inode and dentry cache. You can control the rate at which the kernel reclaims memory used in this cache by adjusting the
vm.vfs_cache_pressure sysctl parameter. This article suggested a value of 10000. As for Linux cache [coherency] itself, I couldn't give you a definitive answer but it's likely not the issue.
unbuffer is for buffering (unbuffered, block, or line-buffering) of the standard streams. It is likely (but not necessarily) not relevant here, unless you are passing tarballs over the net and untarring them locally, or redirecting output from a net-connected process to the filesystem. Even so, it shouldn't matter unless these programs are using block buffering with a huge size, or line-buffering for binary data and you're unlucky enough to not have had a single 0x0d or 0x0a in your data for some arbitrarily large amount of data. Also, this caching is a library feature, not a kernel or program feature. The latter may set parameters.
Netstat would also just list outstanding network connections, not whatever is going on therein.
You can try
strace -e trace=fileing the process to see what it's doing.