If your cluster performance is suffering because of inefficient use of GFS2 caching, you may see large and increasing I/O wait times. You can make use of GFS2's lock dump information to determine the cause of the problem.
The GFS2 lock dump information can be gathered from the debugfs file which can be found at the following path name, assuming that debugfs is mounted on /sys/kernel/debug/:
/sys/kernel/debug/gfs2/fsname/glocks
The content of the file is a series of lines. Each line starting with G: represents one glock, and the following lines, indented by a single space, represent an item of information relating to the glock immediately before them in the file.
The best way to use the debugfs file is to use the cat command to take a copy of the complete content of the file (it might take a long time if you have a large amount of RAM and a lot of cached inodes) while the application is experiencing problems, and then looking through the resulting data at a later date.
It can be useful to make two copies of the debugfs file, one a few seconds or even a minute or two after the other. By comparing the holder information in the two traces relating to the same glock number, you can tell whether the workload is making progress (that is, it is just slow) or whether it has become stuck (which is always a bug and should be reported to Red Hat support immediately).
Lines in the debugfs file starting with H: (holders) represent lock requests either granted or waiting to be granted. The flags field on the holders line f: shows which: The 'W' flag refers to a waiting request, the 'H' flag refers to a granted request. The glocks which have large numbers of waiting requests are likely to be those which are experiencing particular contention.
Table 1.2. Glock flags
| Flag |
Name |
Meaning |
| d |
Pending demote |
A deferred (remote) demote request |
| D |
Demote |
A demote request (local or remote) |
| f |
Log flush |
The log needs to be committed before releasing this glock |
| F |
Frozen |
Replies from remote nodes ignored - recovery is in progress. |
| i |
Invalidate in progress |
In the process of invalidating pages under this glock |
| I |
Initial |
Set when DLM lock is associated with this glock |
| l |
Locked |
The glock is in the process of changing state |
| p |
Demote in progress |
The glock is in the process of responding to a demote request |
| r |
Reply pending |
Reply received from remote node is awaiting processing |
| y |
Dirty |
Data needs flushing to disk before releasing this glock |
Table 1.3. Glock holder flags
| Flag |
Name |
Meaning |
| a |
Async |
Do not wait for glock result (will poll for result later) |
| A |
Any |
Any compatible lock mode is acceptable |
| c |
No cache |
When unlocked, demote DLM lock immediately |
| e |
No expire |
Ignore subsequent lock cancel requests |
| E |
exact |
Must have exact lock mode |
| F |
First |
Set when holder is the first to be granted for this lock |
| H |
Holder |
Indicates that requested lock is granted |
| p |
Priority |
Enqueue holder at the head of the queue |
| t |
Try |
A "try" lock |
| T |
Try 1CB |
A "try" lock that sends a callback |
| W |
Wait |
Set while waiting for request to complete |
Having identified a glock which is causing a problem, the next step is to find out which inode it relates to. The glock number (n: on the G: line) indicates this. It is of the form type/number and if type is 2, then the glock is an inode glock and the number is an inode number. To track down the inode, you can then run find -inum number where number is the inode number converted from the hex format in the glocks file into decimal.
If you run the find on a file system when it is experiencing lock contention, you are likely to make the problem worse. It is a good idea to stop the application before running the find when you are looking for contended inodes.
Table 1.4. Glock types
| Type number |
Lock type |
Use |
| 1 |
Trans |
Transaction lock |
| 2 |
Inode |
Inode metadata and data |
| 3 |
Rgrp |
Resource group metadata |
| 4 |
Meta |
The superblock |
| 5 |
Iopen |
Inode last closer detection |
| 6 |
Flock |
flock(2) syscall |
| 8 |
Quota |
Quota operations |
| 9 |
Journal |
Journal mutex |
If the glock that was identified was of a different type, then it is most likely to be of type 3: (resource group). If you see significant numbers of processes waiting for other types of glock under normal loads, then please report this to Red Hat support.
If you do see a number of waiting requests queued on a resource group lock there may be a number of reason for this. One is that there are a large number of nodes compared to the number of resource groups in the file system. Another is that the file system may be very nearly full (requiring, on average, longer searches for free blocks). The situation in both cases can be improved by adding more storage and using the gfs2_grow command to expand the file system.
