GFS2

GFS2:
=====
    Has the ability to amount the same FS in write mode on more than one node; but there is another mechanisme to track the accessed files and lock it.
Features:
=========
* POSIX : (Portable Operating System Interface based on Unix):
    means support ACL, Quote, Case sensetive
* Reduces the complexity and overhead of managing
* Redundant data copy
* no single point of failure.
* Scalable
* support multiple journals.
    like if we have 2 node cluster we may have two journals. [nummber of journals should be at least the number of nodes using GFS]
    we can add journals later as we may need.

Functions:
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* Native FS.
* Volume management is managed by clvm.
* Support Quota and other POSIX features.

Why Journaling:
===============
* A logical journal stores only changes to file metadata in the journal, and trades fault tolerance for substantially better write performance.
* A file system with a logical journal still recovers quickly after a crash, but may allow unjournaled file data and journaled metadata to fall out of sync with each other, causing data corruption.
    For example, appending to a file may involve three separate writes to:

        1- The file's inode, to note in the file's metadata that its size has increased.
        2- The free space map, to mark out an allocation of space for the to-be-appended data.
        3- The newly allocated space, to actually write the appended data.
    In a metadata-only journal, step 3 would not be logged. If step 3 was not done, but steps 1 and 2 are replayed during recovery, the file will be appended with garbage.

* Modification to the FS is sequentially written to a journal [track all things you did on FS]

Things to consider before building GFS2:
=========================================
* How many GFS filesystems to create initially.
* Unique name for every file system.
* Numbers of journals for GFS FS.
* which node in the cluster will mount GFS FS.
* If you are using GNBD, determine how many GNBD server node are needed.
* determine storage device and partitions to be used for creating logical volumes (clvm) in the FS.

GFS2 Node Locking:
==================
* each node has its own page cache.
* Glock maintaince integrity in cache between nodes.
* GLock uses dlm as the underlying communication layer.
* If glock is configured to be a shared mode (dlm locl mode : PR), then the data under the GLock may be cached on one or more nodes at the same time.
* if the GLock is in exclusive mode (DLM Lock mode : EX), then only a single node may cache data under glock.
* If another node requests a GLock which cannot be granted immediately, then the DLM sends a message to the node or nodes which currently hold the GLock blocking the new request to ask them to drop their lock.


Practise:
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1- install GFS2
2- Load GFS2 module into kernel
    modprobe gfs2
3- Create GFS2 FS and mount it.
    *lv
    *service clvmd status
    *create partition (lv) and format it with gfs2 FS
    *mkfs.gfs2 -h
    *mkfs.gfs2 -j (no of nodes) -p (name of locking protocol) [lock_dlm] -t (name of lock table) [(unique)name of the cluster:name of FS] (partition(lv))
    *Configure fstab on both nodes
    *service gfs2 restart && chkconfig gfs2 on
    *df -h ---> check if it is already mounted autoatically or not.
    *reboot to check every thing is working fine.
    
4- Expand the GFS2.
    *lvextend -L+100M (lv)
    *gfs2_grow (path)
    *df -h
    
5- Enable Quota(on, off, account)
    *man mount.gfs2 --> search for quota.
    *vim /etc/fstab --> add quota=on
    *mount -o remount (mountpoint from fstab)
    *mount |grep gfs2
    *for testing --> on both nodes
        -useradd hazem
        -passwd hazem
        -getent passwd |grep hazem --> id on both should be same if you are not using ldap.
        -gfs2_quota -h
        -gfs2_quota list
        -gfs2_quota sync --> (force it to sync now instead of waitng 60 sec to sync)
        -gfs2_quota limit -m -l 300 -u (userid) -f /gfs2 (mountpoint) --> Hard Limit
        -gfs2_quota warn -m -l 100 -u (userid) -f /gfs2 (mountpoint) --> Soft Limit
        -gfs2_quota list -f /gfs2 (mountpoint)
        -now check if sync done well on the other node
        -create random data to check if it is working fine or not using dd.
        -if no permission we can give him write on it using setfacl --> be sure fstab has acl
        
6- Enabling data journaling, chattr.
    -chattr +j /gfs2 (mountpoint)
    -lsattr /gfs2
    
7- Adding Journal Dynamically.

8- ACL.
    -add acl to fstab
    -setfacl