Editing IO accounting

This page describes accounting for I/O activity of CT processes, on a VFS I/O level.
The feature is available beginning with OpenVZ kernels version <code>028test008</code>.

If you are interested in [[I/O priorities for containers]], it is described in a separate article.

If you are looking for IO scheduler (i.e. lower level) statistics, see [[IO statistics]] instead.

== New resources ==
The following resources are accounted:
; read bytes
: is the amount of bytes read by tasks. Reads are always synchronous in kernel so this type of resources is the easiest one.

; dirty bytes
: is the amount of bytes that were dirty since VE start, i.&nbsp;e. data that isn't yet flushed to disk. This type of resources is accounted using [[RSS_fractions_accounting|page beancounters]] and the context the page was dirtied by is determined like this:
:* if a page is mapped it's "mapper" is used as this may happen in any context (unmapping of a page under memory pressure);
:* if a page is not mapped — current BC is used as this can happen during usual write (writev) only.

; written bytes
: is the amount of bytes flushed to disk. The beancounter to charge this to is the one the page was dirtied by.

; canceled bytes
: is the amount of bytes that were dirty but weren't flushed to disk.

; missed bytes
: is the amount of bytes that were dirtied but the context (beancounter) wasn't saved due to lack of memory.

== Proc interface ==

=== General information ===
As was described in [[BC proc entries|another article]] each beancounter has its own <code>/proc/bc/$BCID</code> directory where subsystems add their entries. I/O accounting adds an <code>ioacct</code> entry to show I/O information. This entry contains the following information:
<pre>
# cat /proc/bc/101/ioacct 
        read                              24330240
        write                               598016
        dirty                               622592
        cancel                               24576
        missed                                   0
        ...
</pre>

Note that <code>dirty</code> is not the size of dirty data at the moment, but the size of dirty data seen so far.

=== Debugging information ===
When <code>CONFIG_UBC_DEBUG_IO</code>, is on <code>/proc/bc/ioacct_debug</code> is added. This entry contains a snapshot of current dirty pages with its beancounter. For example:
<pre>
# cat /proc/bc/ioacct_debug 
Races: io 0 anon 0 clean 0 missed 0
pb         page     flg       cnt     mcnt pb_list  page_pb    mapping  ub      
f7a4a520 e c17cfc68 Dawl        2        0 00000000 f7a4a521   c3870168 0
f7a15ce0 e c17d034c Dawl        2        0 00000000 f7a15ce1   c3870168 0
f72e4680 e c1083364 Dawl        2        0 00000000 f72e4681   c3870168 0
f72e4800 e c1083388 Dawl        2        0 00000000 f72e4801   c3870168 0
f7a15dc0 e c17d0010 Dawl        2        0 00000000 f7a15dc1   c3870168 0
</pre>

== Auxiliary information ==
Along with VFS I/O activity the following information is gathered
; sync counts
: The number of <code>sync(2)</code>, <code>fsync(2)</code>, <code>fdatasync(2)</code> and <code>sync_file_range</code>.
<pre>
# cat /proc/bc/101/ioacct 
        ...
        syncs_total                              0
        fsyncs_total                             0
        fdatasyncs_total                        10
        range_syncs_total                        0
        syncs_active                             0
        fsyncs_active                            0
        fdatasyncs_active                        0
        range_syncs_active                       0
</pre>
<code>_active</code> suffix refers to the number of operations in progress.

; write/read calls counts
: The number of <code>read(2)</code>, <code>readv(2)</code>, <code>write(2)</code>, <code>writev(2)</code>, etc. and the number of bytes passed.
<pre>
# cat /proc/bc/101/ioacct 
        ...
        vfs_reads                            24491
        vfs_read_chars                     2616512
        vfs_writes                             380
        vfs_write_chars                30064899102
</pre>

; number of page beancounters pinned by I/O
: This is the number of page beancounters that save information about page dirtier. This is actually the number of dirty pages within beancounter at the moment.
<pre>
# cat /proc/bc/0/ioacct
        ...
        write                               598016
        dirty                               622592
        ...
        io_pbs                                   0
# dd if=/dev/zero of=tmp bs=512 count=40
# cat /proc/bc/0/ioacct 
        ...
        write                               598016
        dirty                               643072
        ...
        io_pbs                                   5
# sync
# cat /proc/bc/0/ioacct
        ...
        write                               618496
        dirty                               643072
        ...
        io_pbs                                   0
</pre>

== See also ==
* [[IO statistics]]
* [[I/O priorities]]
* [[I/O limits]]

[[Category:Resource management]]