Nfs – in-memory-cache of specific files from NFS

I have seen this kind of behavior when a directory in the PATH is automounted nfs and that directory is unavailable or unresponsive.

I would check the PATH for those accounts. Make sure there is nothing automounted in the path. Then you can add directories one by one and find out which causes the hang.

Another possibility is that you may have some uid mismatch between your accounts from samba and your accounts on the nfs server. When you run 'ls -l' on a users nfs home directory, do you see the correct username? If not you will need to configure Samba to map the usernames to the uids that the nfs server is using.

If you use file-backed memory mapping (e.g. apc.mmap_file_mask=/tmp/apc.XXXXXX) you might see elevated I/O.

Try setting apc.mmap_file_mask to use shared memory (e.g. /apc.shm.XXXXXX) or to /dev/zero (anonymous mmapped memory). Keeping the setting undefined defaults it to using anonymous mmapped memory.

Usually, mmapped files is a great thing:

• Compared to storing something fully in memory, mmapped files usually require less memory
• Compared to saving something to a file, mmapped files require less disk I/O (since writes can be aggregated together).

However, compared to storing something purely in memory, they do incur added I/O - considerably so when the file is continuously changing. The downside of not using mmapped files is a lack of persistence - your cache will not survive a restart, since it is stored only in memory.

One may suggest therefore, that while the cache was filling up and stabilizing, it was undergoing the most change, which had to be constantly written to disk; once the cache was full, the ttl for each object slowed the rate that data in the cache was being turned over, decreasing the change and reducing disk writes.