# AIX sendfile/send_file issues

On AIX, the system call used to send static files with `EnableSendfile
On` is send\_file(). Files sent via send\_file() can be cached by the
kernel in the network buffer cache.

## Problems

### Default tuning of cache size too high

Default AIX tuning of the network buffer cache size is not appropriate
on some AIX systems, and use of send\_file() by IHS can result in large
amounts of kernel memory devoted to the caching of static files. This
may impact other system operations. Because very few applications
utilize the network buffer cache, the use of send\_file() by IBM HTTP
Server may be the the trigger for a problem symptom if the network
buffer cache limits are not appropriate.

How to address this issue if send\_file() needs to be used:

1.  Display the old cache size, with the command `no -o nbc_limit`.
2.  Determine if a new value is appropriate. We recommend that the value
    be no larger than 128MB initially. Choose the current value or
    128MB, whichever is smaller.
3.  If the new value is different, set it with `no -o
    nbc_limit=new_value`.
4.  Monitor actual network buffer cache memory usage with the command
    `netstat -c` during normal web server operation. If the memory usage
    is at or near the configured maximum value, increasing the limit may
    yield better performance. If memory usage is significantly smaller
    than the maximum value, we recommend reducing the maximum value to a
    value closer to actual usage so that the kernel memory usage can't
    increase significantly after this initial monitoring phase has
    completed.

Memory use increases with a larger number of unique files served.

Use the `-p` option of the `no` command to make a permanent change to
the value of `nbc_limit`.

### Delivery of old file contents

The network buffer cache may send old file contents even after the file
has been updated by the user. This will result in the web browser
receiving incorrect file contents.

If the file is now larger than the previously cached content, the client
may receive no response at all, with the following logged in the main
error
log:

``` 
  [Wed Jun 20 08:58:24 2007] [info] [client 127.0.0.1] (22)Invalid argument: core_output_filter: writing data to the network
  
```

If the file is now smaller than the previously cached content, the
client may receive a truncated version of the original contents.

The network buffer cache will check for updates to the file in order to
send the most recent version, but only after a configurable interval has
elapsed. This interval is controlled by the `send_file_duration`
variable which can be displayed or set with the `no` command.

How to address this issue if send\_file() needs to be
used:

### Change the network buffer cache to check for file updates before sending

Set the `send_file_duration` interval to 0; the network buffer cache
will now check for file updates each time the file is sent. *This can
affect other applications on the system which utilize send\_file().* The
default value for `send_file_duration` is 300 (seconds), so without this
configuration change the wrong data could be sent for up to five minutes
after the file update.

Example:

``` 
  no -o send_file_duration=0
```

### Disable the cache temporarily while making file updates

Disable the cache, make the file updates, then restore the cache, as
follows:

1.  Display the old cache size, with the command `no -o nbc_limit`.
2.  Set the size of the network buffer cache to zero temporarily to
    clear the cache, with the command `no -o nbc_limit=0`.
3.  Update the static files.
4.  Restore the previous network buffer cache size, with the command `no
    -o nbc_limit=old_value`, where `old_value` is the value displayed in
    step 1 above.

The set of files to be served with send\_file() can be customized in
order to avoid needing to use the special update mechanism for
infrequently-served files. Examples:

``` 
  # Example 1.  Sendfile is enabled globally, but we disable it
  #             for some rarely-served files in a particular
  #             directory.
  <Directory /www/site/www.example.com/infrequently_served/>
  EnableSendfile Off
  </Directory>

  # Example 2.  Sendfile is disabled globally, but we enable it
  #             for some often-served files.
  <FilesMatch "^(robots\.txt|index\.html|favicon\.ico)$">
  EnableSendfile On
  </FilesMatch>
```

### Can't unmount filesystem

On some levels of AIX, filesystems containing files sent by IHS via
send\_file() can not be unmounted until the network buffer cache is
emptied.

How to address this issue if send\_file() needs to be used:

When unmounting the filesystem containing cached files,

1.  Display the old cache size, with the command `no -o nbc_limit`.
2.  Set the size of the network buffer cache to zero temporarily to
    clear the cache, with the command `no -o nbc_limit=0`.
3.  Unmount the filesystem.
4.  Restore the previous network buffer cache size, with the command `no
    -o nbc_limit=old_value`, where `old_value` is the value displayed in
    step 1 above.

## Other documentation:

  - [AIX 5.3 Command Reference documentation for the `no`
    command](http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.cmds/doc/aixcmds4/no.htm)
  - [FilesMatch](http://publib.boulder.ibm.com/httpserv/manual60/mod/core.html#filesmatch)
  - [EnableSendfile](http://publib.boulder.ibm.com/httpserv/manual60/mod/core.html#enablesendfile)