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This page describes how IO I/O activity of VE processes is accounted. This feature is available since OpenVZ kernels version <code>028test008</code>.
== 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. 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 it was described in [[BC proc entries|another artice]] each beancounter has it's own <code>/proc/bc/$BCID</code> directory where subsystems add their entries. IO I/O accounting adds <code>ioacct</code> entry to show IO I/O information. This entry contains the following information:
<pre>
# cat /proc/bc/101/ioacct
</pre>
Note that <code>dirty</code> is not the size of a dirty data at the moment , but is the size of a 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 in system. E.g.For example:
<pre>
# cat /proc/bc/ioacct_debug
== Auxiliary information ==
Along with VFS IO 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> is accounted.
<pre>
# cat /proc/bc/101/ioacct
; write/read calls counts
: Then 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 is also accounted.
<pre>
# cat /proc/bc/101/ioacct
</pre>
; number of page beancounters pinned by IOI/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>
