Undo retention, flashback query and ora-01555 snapshot too old

Setting up a fault aware database environment is in charge of regarding possible physical and logical error scenarios. On the physical side of the medal, you got real-application-cluster or dataguard at your disposal. The logical one usually comprises backup and replication. Recently, oracle and others introduced features that enables a database to recover from logical errors without or with less remote systems and data. Namely flashback is a powerful technology to look back into (query), back out (transaction) or fully restore (database) the history of data. Flashback of course takes accompanying (history) (meta)data to do its job, namely again, undo-before-images, archived-redo-logs and flashback-logs. So, essentially, the convenience of your course into history depends on the amount of history metadata that is available to the database at the time of a logical error, say an inadvertant delete from.

You may already have learned or heard about or even hardly experienced that flashback-logs may be purged from the recovery-area in favour of archived-redo-logs, rendering your configured flashback-retention-time (DB_FLASHBACK_RETENTION_TARGET) a value of theory. So far, sizing the file-system recovery-area is a task to be performed in a clear-sighted manner. The same is true then for undo-before-images (compare to logs) und the undo-tablespace (compare to area) to accomodate enough data to meet your configured undo-retention-time (UNDO_RETENTION) in UNDO_MANAGEMENT=AUTO mode, while in addition still support long running workload queries (not to overwrite old undo). This article discusses the usability of the (enterprise-manager) undo-advisor in sizing your undo-tablespace to always foster successful flashback queries up to the configured undo-retention-time (as opposed to the nasty surprise of getting an ora-01555, see below, which is also misleading here, imho, in suggesting that the undo-tablespace is to small now, as we know it from workload queries – nope, it has been to small before now, in the past, as regarded to flashback queries – also see : ora-01555 snapshot too old when running flashback query).



Understanding “occurrences before alerts” in oracle enterprise manager 13c

The Incident Manager within Oracle Enterprise Manager 13c is a powerful tool for monitoring a wealth of target types such as databases, hosts, middleware or just services in general from a bird’s eye view or right down into the manifold details of a dedicated application. As with all powerful means around, where there is a lot of power, there also usually is a lot to do wrong (or even employ counterproductive). This is why it is important to understand the Incident Manager concepts from bottom up and be able to identify the knobs and wheels to poke with to meet a certain requirement.

Basically, Incident Manager is a professional toolset to facilitate the management of non-critical and critical system alerts against metric values (registered as problems and incidents, see About Incidents and Problems) in a larger quantity and time scale along with alert management templating and alert assigment and so forth.


Splitting a btrfs 4.1x root partition with debian live system gparted btrfs tools 3.17-x

This is a short picture log of doing a btrfs 4.1x root partition split on a (down) oracle linux 7.2 using a debian live system applying gparted based on btrfs tools 3.17-x. Lot’s of names and version codecs, right? But this is what matters. The important message is : it works using this flavours.
Actually, running oracle or redhat linux as the live system may have been much more appropriate concerning compatibility reasons. The odd things is, no redhat-based (enterprise) linux system features gparted. Only fedora does, sourcing the epel-repository but not having kinf of a live system release as debian.


Tracking down “ntpdate[18168]: no server suitable for synchronization found”

After installing and getting up ntpd on a RHEL-like Linux system, see for example, you may want to double check iff ntpd is actually able to perform successfully. There’s a couple of shell commands to employ, most notably ntpstat and ntpq as well as timedatectl more recently with version 7 and up systems. ntpstat will tell you about the status of the time synchronisation, whereas ntpq, being executed a few times in a row, shows the servers being selected for synchronisation, the one prefixed by an aterisk as the current master, and the time left since the last synchronisation up the the synchronisation interval in the when and poll columns, respectively. timedatectl furthermore includes timezone and daylight saving information, however, the synchronisation status given here is about to be called into question, see later. So far, iff anything runs fine, you may expect something like this (consider the values changing in the when column for the second call):


Locating kernel headers for vmware tools on an uekr3 oracle linux 6.7

Continuing, if you like, on an admin topic concerning UEKR3 Oracle Linux 6.5, see: Missing kernel-firmware 3.8.13-16.2.1.el6uek on Oracle Linux 6.5, I’ going to give a recipe and some explanations for getting vmtools modules successfully built in a VMware guest after complains about an invalid kernel header path.
Immediately after installing vmtools in a VMware guest, usually using /vmware-tools-distrib/, another script, usually /vmware-tools-distrib/bin/ comes up, asking whether you want to configure vmtools just now. Configuration essentially comprises the opt-in/out of functionality as well as building and integration of kernel modules into the running kernel. Iff you furthermore run a recent UEKR3 kernel without the according development packages, the console output may read like this:

Before you can compile modules, you need to have the following installed... 
kernel headers of the running kernel

Search in repoquery --list kernel-uek-devel-3.8.13-68.3.5.el6uek.x86_64 for GCC...
Detected GCC binary at "/usr/bin/gcc".
The path "/usr/bin/gcc" appears to be a valid path to the gcc binary.
Would you like to change it? [no] 

Searching for a valid kernel header path...
The path "" is not a valid path to the 3.8.13-68.3.5.el6uek.x86_64 kernel headers.
Would you like to change it? [yes] y

Enter the path to the kernel header files for the 3.8.13-68.3.5.el6uek.x86_64 kernel

The path "/usr/include/linux" is not a valid path to the 3.8.13-68.3.5.el6uek.x86_64 kernel headers.
Would you like to change it? [yes] n

WARNING: This program cannot compile any modules for the following reason(s)...

- This program could not find a valid path to the kernel headers of the running
kernel.  Please ensure that the header files for the running kernel are 
installed on this sytem.

[ Press Enter key to continue ] 

What actually happens here is quite simple but however also expressed in a misleading way such that she/he may just suppose, installing the kernel headers will fix the problem (I even tried this /usr/include/linux thing, as of the old days, won’t work you see, is none of uekr3 anyway).