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 028test008.
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.
Contents
New resources
The following resources are accounted:
- read bytes
- is the number of bytes read by tasks. Reads are always synchronous in the kernel so this type of resource is the easiest one.
- dirty bytes
- is the number of bytes that were dirty since VE start, i. e. data that isn't yet flushed to disk. This type of resource is accumulated using 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 number of bytes flushed to disk. The beancounter charged with this is the one by which the page was dirtied.
- canceled bytes
- is the number of bytes that were dirty but weren't flushed to disk.
- missed bytes
- is the number 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 another article each beancounter has its own /proc/bc/$BCID directory where subsystems add their entries. I/O accounting adds an ioacct entry to show I/O information. This entry contains the following information:
# cat /proc/bc/101/ioacct
read 24330240
write 598016
dirty 622592
cancel 24576
missed 0
...
Note that dirty is not the size of dirty data at the moment, but the size of dirty data seen so far.
Debugging information
When CONFIG_UBC_DEBUG_IO, is on /proc/bc/ioacct_debug is added. This entry contains a snapshot of current dirty pages with its beancounter. For example:
# 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
Auxiliary information
Along with VFS I/O activity the following information is gathered
- sync counts
- The number of
sync(2),fsync(2),fdatasync(2)andsync_file_range.
# 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
_active suffix refers to the number of operations in progress.
- write/read calls counts
- The number of
read(2),readv(2),write(2),writev(2), etc. and the number of bytes passed.
# cat /proc/bc/101/ioacct
...
vfs_reads 24491
vfs_read_chars 2616512
vfs_writes 380
vfs_write_chars 30064899102
- 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.
# 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