I have one server with two physical disk's, could I install two segment host each one in the separate disk, which means install segment host in the first disk, and another one in the second disk? and if yes, how could I do this?
When init GPDB, you specify the data directories for the segments. Mount each disk on a different mount point, and specify the mount points (or subdirectories) as data dirs.
Additional question: you do have a backup of the database, in case one of the disk fails, right?
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I have an postgresql DB on an AWS Instance, for some reason the instance now is damaged and the only thing I can do is to detach the disk volume and attach it to a new instance.
The challenge I have now is, how do I setup the postgresql DB I had on the damaged instance volume into the new instance without losing any data.
I tried to attach the damaged instance volume as the main volume in the new instance but it doesnt boot up so what I did was that I mounted the volume as a secondary disk and now I can see the information in it including the "data" folder where postgres DB information its supposed to be, but I dont know what to do in order to enable the DB on this new instance.
The files in the /path/to/data/ directory are all that you need in order for a PostgreSQL instance to start up, given that the permissions are set to 0700 and owned by the user starting up the process (usually postgres, but sometimes others). The other things to bear in mind are:
Destination OS needs to be the same as where you got the data/ directory from (because filesystem variations may either corrupt your data or prevent Postgres from starting up)
Destination filesystem needs to be the same as where you got your data/ directory from (for reasons similar to above)
Postgres major version needs to be the same as where you got your data/ directory from (for reasons similar to above)
If these conditions are met, you should be able to bring up the database and use it.
In many of the SQL Server articles it is mentioned that the best practice is to use multiple File group on a physical disk to avoid disk contention and disk spindle issues .So my query are :
1:Does the same theory of having multiple file group hold true for
a virtual machine ?
2:Should i still create my temp db to a
different disk and should i also create multiple temp db files to
avoid large read/write operation on the same temp db file in a
virtual machine setup for my production environment
You recommendation and reasoning would be helpful to decide the best practice.
Thanks.
Yes, it still applies to virtual servers. Part of the contention problem is accessing the Global Allocation Map (GAM) or Shared Global Allocation Map (SGAM), which exists for each database file and can only be accessed by one process or thread at a time. This is the "latch wait" problem.
If your second disk is actually on different spindles, then yes. If the database files would be on different logical disks but identical spindles, then it's not really important.
The MS recommendation is that you should create one database file for each logical processor on your server, up to 8. You should test to see if you find problems with latch contention on tempdb before adding more than 8 database files.
You do not need to (and generally should not) create multiple tempdb log files because those are used sequentially. You're always writing to the next page in the sequence, so there's no way to split up disk contention.
The question needs a bit more information about your environment. If the drives for the VM are hosted on a SAN somewhere and the drives presented to the VM are all spread across the same physical disks on the SAN then you're not going to avoid contention. If, however, the drives are not on the same physical drives then you may see an improvement. Your SAN team will have to advise you on that.
That being said, I still prefer having the log files split from the data file and tempDB being on it's own drive. The reason being that if a query doesn't go as planned then it can fill the log file drive which may take that database offline, but other databases may still be able to keep running (assuming they have enough empty space in their log files).
Again with tempDB, if that does get filled then the transaction will error out, and everything else should keep running without intervention.
I have 3 instances of SQL Server 2008, each on different machines with multiple databases on each instance. I have 2 separate LUNS on my SAN for MDF and LDF files. The NDX and TempDB files run on the local drive on each machine. Is it O.K. for the 3 instances to share a same volume for the data files and another volume for the log files?
I don't have thin provisioning on the SAN so I would like to not constaint disk space creating multiple volumes because I was adviced that I should create a volume (drive letter) for each instance, if not for each database. I am aware that I should split my logs and data files at least. No instance would share the actual database files, just the space on drive.
Any help is appretiated.
Of course the answer is: "It depends". I can try to give you some hints on what it depends however.
A SQL Server Instance "assumes" that it has exclusive access to its resources. So it will fill all available RAM per default, it will use all CPUs and it will try to saturate the I/O channels to get maximum performance. That's the reason for the general advice to keep your instances from concurrently accessing the same disks.
Another thing is that SQL Server "knows" that sequential I/O access gives you much higher trhoughput than random I/O, so there are a lot of mechanisms at work (like logfile organization, read-ahead, lazy writer and others) to avoid random I/O as much as possible.
Now, if three instances of SQL Server do sequential I/O requests on a single volume at the same time, then from the perspective of the volume you are getting random I/O requests again, which hurts your performance.
That being said, it is only a problem if your I/O subsystem is a significant bottleneck. If your logfile volume is fast enough that the intermingled sequential writes from the instances don't create a problem, then go ahead. If you have enough RAM on the instances that data reads can be satisfied from the buffer cache most of the time, you don't need much read performance on your I/O subsystem.
What you should avoid in each case is multiple growth steps on either log or data files. If several files on one filesystem are growing, you will get fragmentation and fragmentation can transform a sequential read or write request even from a single source to random I/O again.
The whole picture changes again if you use SSDs as disks. These have totally different requirements and behaviour, but since you didn't say anything about SSD I will assume that you use a "conventional" disk-based array or RAID configuration.
Short summary: You might get away with it, if the circumstances are right, but it is hard to assess without knowing a lot more about your systems, from both the SAN and SQL perspective.
Can anyone think of a reason a SQL Server 2008 failover cluster couldn't use Cluster Shared Volumes for databases and log files?
It seems that using CSVs should reduce failover time and reduce the complexity of the cluster group configurations (the physical drive resources wouldn't need to "failover" anymore).
I think, but am not 100% sure, Microsoft restricts what can be on a CSV.
Of course, you can put the VHD there. That's the whole point. And the VHD could contain the database data and log files -- who knows what's "inside" a VHD.
But, not knowing your configuration, I don't know what you are trying to do. CSV is there so a single volume (e.g., a single LUN on a SAN) can be shared by multiple cluster members with individual files there in being used by different cluster members. Specifically, the information making up a VM definition and VHD.
Previously, one had to put the VM definition and VHD in a separate LUN so it could move about individually. There was nothing "wrong" with this other than the complexity of having so many LUNs.
Database files are different. You don't have as many. They are big. You want to carefully place them and watch them. Etc.
If you just put the database files inside a VHD then, as said originally, all is easy, except you don't get the detailed treatment you probably want.
If you put the database files in a separate LUN then you have all of the detailed treatment and that LUN will failover as easily as anything...
I'm looking for a way to quickly "clone" a database from 1 postgresql server to another.
Assuming...
I have a postgresql server running on HostA, serving 2 databases
I have 2 devices mounted on HostA, each device stores the data for one of the database (i.e. 1 database => 1 tablespace => 1 device )
I am able to obtain a "safe" point in time snapshot through some careful method
I am able to consistently and safely produce a clone of any of the 2 devices used (I could even assume both database only ever receive reads)
The hosts involved are CentOS 5.4
Is it possible to host a second postgresql server on HostB, mount one of the cloned devices and get the corresponding database to "pop" into existence? (Kind of like when you copy the MyISAM table files in MySQL).
If this is at all possible, what is the mechanism (i.e. what DDL should I look into or pg commands)?
It's important for me to be able to move individual databases in isolation of each other. But if this isn't possible, would a similar approach work at a server level (trying to clone and respawn a server by copying the datadir over to a host with the same postgresql install) ?
Not easily, because there are a number of files that are shared between databases which means each database in the same install is dependent on this.
You can do it at the server level, or at the cluster level, but not at individual database level. Just be sure to copy/clone over the whole data directory and all external tablespaces. As long as you can make the clone atomically (either on the same filesystem or with a system that can do atomic clones across filesystems), you don't even need to stop the database on hostA to do it.