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doc: Introduction to meta xlator.

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... in the form of examples. This is by no means exhaustive.  Meta
xlator is not as well known as it should be for its own good.  This
patch is to get the ball rolling.

Change-Id: I5898c37aa15706e886af68f19caac48c175df90b
BUG: 1144282
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/8410
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
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Krishnan Parthasarathi 2014-08-04 16:27:52 +05:30 committed by Vijay Bellur
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Meta translator
===============
Introduction
------------
Meta xlator provides an interface similar to the Linux procfs, for GlusterFS
runtime and configuration. This document lists some useful information about
GlusterFS internals that could be accessed via the meta xlator. This is not
exhaustive at the moment. Contributors are welcome to improve this.
Note: Meta xlator is loaded automatically in the client graph, ie. in the
mount process' graph.
### GlusterFS native mount version
>[root@trantor codebase]# cat $META/version
>{
> "Package Version": "3.7dev"
>}
### Listing of some files under the `meta` folder
>[root@trantor codebase]# mount -t glusterfs trantor:/vol /mnt/fuse
>[root@trantor codebase]# ls $META
>cmdline frames graphs logging mallinfo master measure_latency process_uuid version
### GlusterFS' process identifier
>[root@trantor codebase]# cat $META/process_uuid
>trantor-11149-2014/07/25-18:48:50:468259
>
This identifier appears in connection establishment log messages.
For eg.,
>[2014-07-25 18:48:49.017927] I [server-handshake.c:585:server_setvolume] 0-vol-server: accepted client from trantor-11087-2014/07/25-18:48:48:779656-vol-client-0-0-0 (version: 3.7dev)
>
### GlusterFS command line
>[root@trantor codebase]# cat $META/cmdline
>{
> "Cmdlinestr": "/usr/local/sbin/glusterfs --volfile-server=trantor --volfile-id=/vol /mnt/fuse"
>}
### GlusterFS volume graph
The following directory structure reveals the way xlators are stacked in a
graph like fashion. Each (virtual) file under a xlator directory provides
runtime information of that xlator. For eg. 'name' contains the name of the
xlator.
```
/mnt/fuse/.meta/graphs/active
|-- meta-autoload
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol
| |-- type
| `-- view
|-- top -> meta-autoload
|-- vol
| |-- history
| |-- meminfo
| |-- name
| |-- options
| | |-- count-fop-hits
| | `-- latency-measurement
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-md-cache
| |-- type
| `-- view
|-- vol-client-0
| |-- history
| |-- meminfo
| |-- name
| |-- options
| | |-- client-version
| | |-- clnt-lk-version
| | |-- fops-version
| | |-- password
| | |-- ping-timeout
| | |-- process-uuid
| | |-- remote-host
| | |-- remote-subvolume
| | |-- send-gids
| | |-- transport-type
| | |-- username
| | |-- volfile-checksum
| | `-- volfile-key
| |-- private
| |-- profile
| |-- subvolumes
| |-- type
| `-- view
|-- vol-client-1
| |-- history
| |-- meminfo
| |-- name
| |-- options
| | |-- client-version
| | |-- clnt-lk-version
| | |-- fops-version
| | |-- password
| | |-- ping-timeout
| | |-- process-uuid
| | |-- remote-host
| | |-- remote-subvolume
| | |-- send-gids
| | |-- transport-type
| | |-- username
| | |-- volfile-checksum
| | `-- volfile-key
| |-- private
| |-- profile
| |-- subvolumes
| |-- type
| `-- view
|-- vol-dht
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | |-- 0 -> ../../vol-client-0
| | `-- 1 -> ../../vol-client-1
| |-- type
| `-- view
|-- volfile
|-- vol-io-cache
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-read-ahead
| |-- type
| `-- view
|-- vol-md-cache
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-open-behind
| |-- type
| `-- view
|-- vol-open-behind
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-quick-read
| |-- type
| `-- view
|-- vol-quick-read
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-io-cache
| |-- type
| `-- view
|-- vol-read-ahead
| |-- history
| |-- meminfo
| |-- name
| |-- options
| |-- private
| |-- profile
| |-- subvolumes
| | `-- 0 -> ../../vol-write-behind
| |-- type
| `-- view
`-- vol-write-behind
|-- history
|-- meminfo
|-- name
|-- options
|-- private
|-- profile
|-- subvolumes
| `-- 0 -> ../../vol-dht
|-- type
`-- view
```