Pat Shuff's Blog

resizing database as a service with Oracle

Historically, what happens to a database months after deployment has always been an issue and problem. If we go out and purchase a computer and disk storage then deploy a database onto the server. If we oversize the hardware and storage, we wasted budget. If we undersize the hardware and storage we had to purchase a new computer or new storage and get an operating system expert to reconfigure everything on the new server and get a database administrator to reconfigure the database installation to run on the new server or new storage. For example, if we purchased a 1 TB disk drive and allocated it all to /u02 the database had a ton of space to grow into. We put the DATA area there and put the RECO area into /u03. Our database service suddenly grows wildly and we have a record number of transactions and increase the offerings in our product catalog and our tablespace suddenly grows to over 800 GB. Disk performance starts to suffer and we want to grow our 1 TB to 2 TB. To do this we have to shut down our database, shut down the operating system, attach the new disk, format and mount it as /u05, copy the data from /u02 to /u05, remount /u05 as /u02, and reboot the system. We could have backed up the database from /u02 and reformatted /u02 and /u05 as a logical volume to allow us to dynamically grow the disk and allow us to purchase a 1 TB for our /u05 disk rather than a 2 TB disk and reduce our cost. We successfully grew our tablespace by purchasing more hardware, involving an operating system admin, and our database administrator. We were only down for a day or half day while we copied all of our data and modified the disk layout.

Disk vendors attacked this problem early by offering network or fiber attached storage rather that direct attached storage. They allow you to add disks dynamically keeping you from having to go out and purchase new disks. You can attach your disk as a logical unit number and add spindles as desired. This now requires you to get a storage admin involved to update your storage layout and grow your logical unit space from 1 TB to 2 TB. You then need to get your operating system admin to grow the file system that is on your /u02 logical unit mount to allow your database admin to grow the tablespace beyond the 1 TB boundary. Yes, this solves the problem of having to bring down the server, touch the hardware, add new cables and spindles to the computer. It allows data centers to be remote and configurations to be done dynamically with remote management tools. It also addresses the issue of disk failures much easier and quicker by pushing the problem to the storage admin to monitor and fix single disk issues. It solves a problem but there are better ways today to address this issue.

With infrastructure as a service we hide these issues by treating storage in the cloud as dynamic storage. With Amazon we can provision our database in EC2 and storage in S3. If we need to grow our S3, we allocate more storage to our bucket and grow the file system in EC2. The database admin then needs to go in and grow the tablespace to fill the new storage area. We got rid of the need for a storage admin, reduced our storage cost, and eliminated a step in our process. We still need an operating system admin to grow the file system and a database admin to grow the tablespace. The same is true if we use Azure compute or Oracle IaaS.

Let's go through how to attach and grow storage to a generic compute instance. We have a CentOS image running in IaaS on the Oracle Cloud. We can see that the instance has 9 GB allocated to it as the root operating system. We would like to add a 20 GB disk then grow the disk to 40 GB as a second test. At first we notice that our instance is provisioned and we the 9 GB disk labeled CentOS7 allocated to our instance as /dev/xvdb. We then create a root partition /dev/xvdb1, provision an operating system onto it using the xfs file system, and mount it as the root filesystem.

To add a 20 GB disk, we go into the Compute management screen, and create a new storage volume. This is easy because we just create a new volume and allocate 20 GB to it.

Given that this disk is relatively small, we don't have to wait long and can then attach it to our CentOS7 instance by clicking on the hamburger menu to the right of our new 20 GB disk and attaching it to our CentOS7 instance.

It is important to note that we did not need to reboot the instance but suddenly the disk appears as /dev/xvdc. We can then partition the disk with fdisk, create a file system with mkfs, and mount the disk by creating a new /u02 mount point and mounting /dev/xvdc1 on /u02.

The real exercise here is to grow this 20 GB mounted disk to 40 GB. We can go into the Volume storage and Update the storage to a larger size. This is simple and does not require a reboot or much work. We go to the Storage console, Update the disk, grow it to 40 GB, and go back to the operating system and notice that our 20 GB disk is now 40 GB. We can create a new partition /dev/xvdc2 and allocate it to our storage.

Note that we selected poorly when we made our file system selection. We selected to lay out an ext3 file system onto our /dev/xvdc1 partition. We can't grow the ext3 filesystem. We should have selected ext4. We did this on purpose to prove a point. The file selection is critical and if you make the wrong choice there is no turning back. The only way to correct this is to get a backup of our /u02 mount and restore it onto the ext4 newly formatted partition. We also made a second wrong choice of laying the file system directly on the raw partition. We really should have created a logical partition from this one disk and put the file system on the logical partition. This would allow us to take our new /dev/xvdc2, create a new physical partition, add the physical partition to our logical partition, and grow the ext4 file system. Again, we did this on purpose to prove a point. You need to plan on expansion when you first lay out a system. To solve this problem we need to unmount the /u02 disk, delete the /dev/xvdc1 and /dev/xvdc2 partitions, create a physical partition with logical volume manager, create a logical partition, and lay an ext4 file system onto this new volume. We then restore our data from the backup and can simply grow the partition much easier in the future. We are not going to go through these steps because the exercise is to show you that it is much easier with platform as a service and not how to do it on infrastructure as a service.

If we look at a database as a service disk layout we notice that we have /dev/xvdc1 as /u01 which represents the ORACLE_HOME, /dev/mapper/dataVolGroup-lvol0 as /u02 which represents the tablespace area for the database, /dev/mapper/fraVolGroup-lvol0 which represents the fast recovery area (where RMAN dumps backups), and /dev/mapper/redoVolGroup-lvol0 which represents the redo log area (where DataGuard dumps the transactions logs). The file systems are logical volumes and created by default for us. The file systems are ext4 which can be seen by looking at the /etc/fstab file. If we need to grow the /u02 partition we can do this by using the scale up option for the database. We can add 20 GB and extend the data partition or the fra partition. We also have the option of attaching the storage as /u05 and manually growing partitions as desired. It is important to note that scaling up the database does require a reboot and restart of the database. When we try to scale up this database instance we get a warning that there is a Java service that depends upon the database and it must be stopped before we can add the storage desired.

In summary, we can use IaaS to host a database. It does get rid of the need for a storage administrator. It does not get rid of the need for an operating system administrator. We still have to know the file system and operating system commands. If we use PaaS to host a database, we can add storage as a database administrator and not need to mess with the logical volume or file system commands. We can grow the file system and add table extents quickly and easily. If we undersize our storage, correcting for this mistake is much easier than it was years ago. We don't need to overpurchase storage anymore because we can allocate it on demand and pay for the storage as we use it. We can easily remove one of the headaches that has been an issue for years and no longer need to triple our storage estimates and go with realistic estimates and control budget better and easier.

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