=head1 NAME

fs_storebehind - Enables asynchronous writes to the file server

=head1 SYNOPSIS

=for html
<div class="synopsis">

B<fs storebehind> S<<< [B<-kbytes> <I<asynchrony for specified names>>] >>>
    S<<< [B<-files> <I<specific pathnames>>+] >>>
    S<<< [B<-allfiles> <I<new default (KB)>>] >>> [B<-verbose>] [B<-help>]

B<fs st> S<<< [B<-k> <I<asynchrony for specified names>>] >>>
    S<<< [B<-f> <I<specific pathnames>>+] >>>
    S<<< [B<-a> <I<new default (KB)>>] >>> [B<-v>] [B<-h>]

=for html
</div>

=head1 DESCRIPTION

The B<fs storebehind> command enables the Cache Manager to perform a
delayed asynchronous write to the File Server when an application closes a
file. By default, the Cache Manager writes all data to the File Server
immediately and synchronously when an application program closes a file --
that is, the close() system call does not return until the Cache Manager
has actually transferred the final chunk of the file to the File
Server. This command specifies the number of kilobytes of a file that can
still remain to be written to the File Server when the Cache Manager
returns control to the application. It is useful if users working on the
machine commonly work with very large files, but also introduces the
complications discussed in the L<CAUTIONS>.

Set either or both of the following in a single command:

=over 4

=item *

To set a value that applies to all AFS files manipulated by applications
running on the machine, use the B<-allfiles> argument. This value is
termed the I<default store asynchrony> for the machine, and persists until
the machine reboots. If it is not set, the default value is zero,
indicating that the Cache Manager performs synchronous writes.

As an example, the following setting means that when an application closes
a file, the Cache Manager can return control to the application as soon as
no more than 10 kilobytes of the file remain to be written to the File
Server.

   -allfiles 10

=item *

To set a value that applies to one or more individual files, and overrides
the value of the B<-allfiles> argument for them, combine the B<-kbytes>
and B<-files> arguments. The setting persists as long as there is an entry
for the file in the kernel table that the Cache Manager uses to track
certain information about files. In general, such an entry persists at
least until an application closes the file or exits, but the Cache Manager
is free to recycle the entry if the file is inactive and it needs to free
up slots in the table. To increase the certainty that there is an entry
for the file in the table, issue the B<fs storebehind> command shortly
before closing the file.

As an example, the following setting means that when an application closes
either of the files B<bigfile> and B<biggerfile>, the Cache Manager can
return control to the application as soon as no more than a megabyte of
the file remains to be written to the File Server.

   -kbytes 1024 -files bigfile biggerfile

Note that once an explicit value has been set for a file, the only way to
make it subject to the default store asynchrony once again is to set
B<-kbytes> to that value. In other words, there is no combination of
arguments that automatically makes a file subject to the default store
asynchrony once another value has been set for the file.

=back

To display the settings that currently apply to individual files or to all
files, provide the command's arguments in certain combinations as
specified in L<OUTPUT>.

=head1 CAUTIONS

For the following reasons, use of this command is not recommended in most
cases.

In normal circumstances, an asynchronous setting results in the Cache
Manager returning control to applications earlier than it otherwise does,
but this is not guaranteed.

If a delayed write fails, there is no way to notify the application, since
the close() system call has already returned with a code indicating
success.

Writing asynchronously increases the possibility that the user will not
notice if a write operation makes the volume that houses the file exceed
its quota. As always, the portion of the file that exceeds the volume's
quota is lost, which prompts a message such as the following:

   No space left on device

To avoid losing data, it is advisable to verify that the volume housing
the file has space available for the amount of data anticipated to be
written.

=head1 OPTIONS

=over 4

=item B<-kbytes> <I<asynchrony for specified names>>

Specifies the number of kilobytes of data from each file named by the
B<-files> argument that can remain to be written to the file server when
the Cache Manager returns control to an application program that closed
the file. The B<-files> argument is required along with this
argument. Provide an integer from the range C<0> (which reinstates the
Cache Manager's default behavior or writing synchronously) to the maximum
AFS file size.

=item B<-files> <I<specific pathnames>>+

Names each file to which the value set with the B<-kbytes> argument
applies. The setting persists as long as there is an entry for the file in
the kernel table that the Cache Manager uses to track certain information
about files. Because closing a file generally erases the entry, when
reopening a file the only way to guarantee that the setting still applies
is to reissue the command. If this argument is provided without the
B<-kbytes> argument, the command reports the current setting for the
specified files, and the default store asynchrony.

=item B<-allfiles> <I<new default (KB)>>

Sets the default store asynchrony for the local machine, which is the
number of kilobytes of data that can remain to be written to the file
server when the Cache Manager returns control to the application program
that closed a file. The value applies to all AFS files manipulated by
applications running on the machine, except those for which settings have
been made with the B<-kbytes> and B<-files> arguments. Provide an integer
from the range C<0> (which indicates the default of synchronous writes) to
the maximum AFS file size.

=item B<-verbose>

Produces output confirming the settings made with the accompanying
B<-kbytes> and B<-files> arguments, the B<-allfiles> argument, or all
three. If provided by itself, reports the current default store
asynchrony.

=item B<-help>

Prints the online help for this command. All other valid options are
ignored.

=back

=head1 OUTPUT

If none of the command's options are included, or if only the B<-verbose>
flag is included, the following message reports the default store
asynchrony (the setting that applies to all files manipulated by
applications running on the local machine and for which not more specific
asynchrony is set).

   Default store asynchrony is <x> kbytes.

A value of C<0> (zero) indicates synchronous writes and is the default if
no one has included the B<-allfiles> argument on this command since the
machine last rebooted.

If the B<-files> argument is provided without the B<-kbytes> argument, the
output reports the value that applies to each specified file along with
the default store asynchrony. If a particular value has previously been
set for a file, the following message reports it:

   Will store up to <y> kbytes of <file> asynchronously.
   Default store asynchrony is <x> kbytes.

If the default store asynchrony applies to a file because no explicit
B<-kbytes> value has been set for it, the message is instead as follows:

   Will store <file> according to default.
   Default store asynchrony is <x> kbytes.

If the B<-verbose> flag is combined with arguments that set values
(B<-files> and B<-kbytes>, or B<-allfiles>, or all three), there is a
message that confirms immediately that the setting has taken effect. When
included without other arguments or flags, the B<-verbose> flag reports
the default store asynchrony only.

=head1 EXAMPLES

The following command enables the Cache Manager to return control to the
application program that closed the file F<test.data> when 100 kilobytes
still remain to be written to the File Server. The B<-verbose> flag
produces output that confirms the new setting, and that the default store
asynchrony is zero.

   % fs storebehind -kbytes 100 -files test.data -verbose
   Will store up to 100 kbytes of test.data asynchronously.
   Default store asynchrony is 0 kbytes.

=head1 PRIVILEGE REQUIRED

To include the B<-allfiles> argument, the issuer must be logged in as the
local superuser C<root>.

To include the B<-kbytes> and B<-files> arguments, the issuer must either
be logged in as the local superuser C<root> or have the C<w> (write)
permission on the ACL of each file's directory.

To view the current settings (by including no arguments, the B<-file>
argument alone, or the B<-verbose> argument alone), no privilege is
required.

=head1 SEE ALSO

L<afsd(8)>

=head1 COPYRIGHT

IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved.

This documentation is covered by the IBM Public License Version 1.0.  It was
converted from HTML to POD by software written by Chas Williams and Russ
Allbery, based on work by Alf Wachsmann and Elizabeth Cassell.