David Teigland 5d37b26270 label_scan/vg_read: use label_read_data to avoid disk reads ...

The new label_scan() function reads a large buffer of data
from the start of the disk, and saves it so that multiple
structs can be read from it.  Previously, only the label_header
was read from this buffer, and the code which needed data
structures that immediately followed the label_header would
read those from disk separately.  This created a large
number of small, unnecessary disk reads.

In each place that the two read paths (label_scan and vg_read)
need to read data from disk, first check if that data is
already available from the label_read_data buffer, and if
so just copy it from the buffer instead of reading from disk.

Code changes
------------

- passing the label_read_data struct down through
  both read paths to make it available.

- before every disk read, first check if the location
  and size of the desired piece of data exists fully
  in the label_read_data buffer, and if so copy it
  from there.  Otherwise, use the existing code to
  read the data from disk.

- adding some log_error messages on existing error paths
  that were already being updated for the reasons above.

- using similar naming for parallel functions on the two
  parallel read paths that are being updated above.

  label_scan path calls:
  read_metadata_location_summary, text_read_metadata_summary

  vg_read path calls:
  read_metadata_location_vg, text_read_metadata_file

  Previously, those functions were named:

  label_scan path calls:
  vgname_from_mda, text_vgsummary_import

  vg_read path calls:
  _find_vg_rlocn, text_vg_import_fd

I/O changes
-----------

In the label_scan path, the following data is either copied
from label_read_data or read from disk for each PV:

- label_header and pv_header
- mda_header (in _raw_read_mda_header)
- vg metadata name (in read_metadata_location_summary)
- vg metadata (in config_file_read_fd)

Total of 4 reads per PV in the label_scan path.

In the vg_read path, the following data is either copied from
label_read_data or read from disk for each PV:

- mda_header (in _raw_read_mda_header)
- vg metadata name (in read_metadata_location_vg)
- vg metadata (in config_file_read_fd)

Total of 3 reads per PV in the vg_read path.

For a common read/reporting command, each PV will be:

- read by the command's initial lvmcache_label_scan()
- read by lvmcache_label_rescan_vg() at the start of vg_read()
- read by vg_read()

Previously, this would cause 11 synchronous disk reads per PV:
4 from lvmcache_label_scan(), 4 from lvmcache_label_rescan_vg()
and 3 from vg_read().

With this commit's optimization, there are now 2 async disk reads
per PV: 1 from lvmcache_label_scan() and 1 from
lvmcache_label_rescan_vg().

When a second mda is used on a PV, it is located at the
end of the PV.  This second mda and copy of metadata will
not be found in the label_read_data buffer, and will always
require separate disk reads.

2017-10-31 14:58:00 -05:00

..

label.c

label_scan/vg_read: use label_read_data to avoid disk reads

2017-10-31 14:58:00 -05:00

label.h

label_scan/vg_read: use label_read_data to avoid disk reads

2017-10-31 14:58:00 -05:00