Overview

The lvcache command helps you create and manage LVM cache devices.

Installation

This is a standard Python package, you can run:

python setup.py install

Or:

pip install .

Or you can even install it directly from GitHub:

pip install git+http://github.com/larsks/lvcache.git

Global options

• --human, -H

  Display volume sizes using SI suffixes (e.g., 1M = 1000K = 1000000B).

Listing information about existing LVs

lvcache list [-h] [-f {csv,table}] [-c COLUMN] [--max-width <integer>]
             [--quote {all,minimal,none,nonnumeric}] [--all]
             vgname

List LVs on VG tank:

# lvcache list tank
+---------+-----------+------+
| name    | is_cached | size |
+---------+-----------+------+
| home    | False     | 157G |
| root    | True      | 53G  |
| vol0    | False     | 2G   |
+---------+-----------+------+

Creating and attaching a cache volume

lvcache create [-h] [--cache-percent CACHE_PERCENT]
               [--cache-device CACHE_DEVICE]
               [--cache-tag CACHE_TAG] [--cache-mode CACHE_MODE]
               lvspec

• --cache-percent, -% percent

  Set the size of the cache LV as a percentage of the size of the origin LV.

  Defaults to 20%.

• --cache-tag, -t tag

  Places cache and metadata LV on PV identified by tag tag. You can see tags associated with PVs by running:

  pvs -o+tags
  

  You can apply a tag to a PV with the pvchange command:

  pvchange --addtag <tag> /dev/sdx
  

  Defaults to cache.

• --cache-device, -d device

  Please cache and metadata LV on specific a specific PV identified by a device path (e.g., /dev/sda).

• --cache-mode, -m mode

  Sets cache mode for cache LV. Can be either writeback or writethrough. NB: This option may not be implemented in LVM at this time.

  The default mode is writethrough.

Create a cache LV and attach it to tank/home:

# lvcache create tank/home
creating 8388608 byte metadata LV
creating 6442450944 byte cache LV
attaching cache LV tank/home to data LV tank/home_cache

Detaching and removing a cache LV

lvcache remove [-h] lvspec

Remove the cache LV associated with tank/home:

# lvcache remove tank/home
removing cache LV tank/home_cache
Flushing cache for home.
534 blocks must still be flushed.
0 blocks must still be flushed.

Getting the status of a cached LV

lvcache status [-h] [-f {shell,table,value}] [-c COLUMN]
               [--max-width <integer>]
               [--variable VARIABLE] [--prefix PREFIX]
               lvspec

Display status of the home LV:

# lvcache status -H tank/home
+-----------------------+------------------+
| Field                 | Value            |
+-----------------------+------------------+
| cached                | True             |
| size                  | 32G              |
| cache_lv              | home_cache       |
| cache_lv_size         | 6G               |
| metadata_lv           | home_cache_cmeta |
| metadata_lv_size      | 8M               |
| cache_block_size      | 128              |
| cache_utilization     | 0/98304          |
| cache_utilization_pct | 0.0              |
| demotions             | 0                |
| dirty                 | 0                |
| end                   | 62914560         |
| features              | 1                |
| md_block_size         | 8                |
| md_utilization        | 200/2048         |
| md_utilization_pct    | 9.765625         |
| promotions            | 0                |
| read_hits             | 0                |
| read_misses           | 0                |
| segment_type          | cache            |
| start                 | 0                |
| write_hits            | 0                |
| write_misses          | 0                |
+-----------------------+------------------+

Because lvcache is using the cliff framework, it is very easy to extract individual values from this list for graphing or monitoring purposes:

# lvcache status tank.home -f value -c md_utilization_pct
9.765625

Or:

# lvcache status tank.home -f shell
cached="True"
size="32G"
cache_lv="nova_cache"
cache_lv_size="6G"
metadata_lv="nova_cache_cmeta"
metadata_lv_size="8M"
cache_block_size="128"
cache_utilization="0/98304"
cache_utilization_pct="0.0"
demotions="0"
dirty="0"
end="62914560"
features="1"
md_block_size="8"
md_utilization="200/2048"
md_utilization_pct="9.765625"
promotions="0"
read_hits="0"
read_misses="0"
segment_type="cache"
start="0"
write_hits="0"
write_misses="0"

License

lvcache, a program for managing LVM cache devices
Copyright (C) 2014 Lars Kellogg-Stedman lars@oddbit.com

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.