This is very cool:
There are a bunch of nice computer science classes online from mit: Free online MIT computer science classes
Here is an introductory computer science class: Intro to computer science
Here is a graded version of the same class on edX: Graded version of MIT intro CS class starting June 10th.
edX does not have as many computer science classes but edX may motivate students because edX classes include grades and certificates for those who (more...)
I dug up a simple C program that I wrote years ago to test disk performance. I hesitated to publish it because it is rough and limited in scope and other more capable tools exist. But, I have made good use of it so why not share it with others? It takes a file name and the size of the file in megabytes. It sequentially writes the file in 64 kilobyte chunks. It opens the (more...)
In my previous post I described how I could not explain why I got better db file parallel read wait times in a test on Linux than I got running the same test on HP-UX. I have discovered that the Linux wait times were better because Linux cached the data in the filesystem cache and HP-UX did not.
Neither system used direct I/O for the tests so both could cache data in the filesystem cache. (more...)
I am still working on comparing performance between an HP-UX blade and a Linux virtual machine and I have a strange result. I tried to come up with a simple example that would do a lot of single block I/O. The test runs faster on my Linux system than my HP-UX system and I’m not sure why. All of the parameters are the same, except the ones that contain the system name and filesystem names. (more...)
I’m trying to compare two types of database servers and it looks like one has a faster CPU than the other. But, the benchmark I have used runs a complicated variety of SQL so it is hard to really pin down the CPU performance. So, I made up a simple query that eats up a lot of CPU and does not need to read from disk.
First I created a small table with five rows:
I need to change a view and an index on an active production system. I’m concerned that the change will fail with a “ORA-00054: resource busy” error because I’m changing things that are in use. I engaged in a twitter conversation with @FranckPachot and @DBoriented and they gave me the idea of using DDL_LOCK_TIMEOUT with a short timeout to sneak in my changes on our production system. Really, I’m more worried about backing out the changes since (more...)
I’m trying to compare how a query runs on two different 22.214.171.124 systems. One runs on HP-UX Itanium and one runs on 64 bit x86 Linux. Same query, same plan, different hash value.
update seg$ set type#=:4,blocks=:5,extents=:6,minexts=:7,maxexts=:8,exts
ize=:9,extpct=:10,user#=:11,iniexts=:12,lists=decode(:13, 65535, NULL,
:13),groups=decode(:14, 65535, NULL, :14), cachehint=:15, hwmincr=:16,
spare1=DECODE(:17,0,NULL,:17),scanhint=:18, bitmapranges=:19 where
ts#=:1 and file#=:2 and block#=:3
Plan hash value: 1283625304
| Id | Operation | Name | Rows | Bytes (more...)
I started using sar -d to look at disk performance on a Linux system this week and had to look up what some of the returned numbers meant. I’ve used sar -d on HP Unix but the format is different.
Here is an edited output from a Linux VM that we are copying files to:
$ sar -d 30 1
Linux 2.6.32-504.3.3.el6.x86_64 (myhostname) 04/01/2015 _x86_64_ (4 CPU)
05:26:55 PM (more...)
I was preparing for my weekend patch of our Exadata system and I needed to back up all of our Oracle homes and inventories on our production system. On our 2 node dev and qa clusters I just ran the backups by hand like this:
login as root
tar -cvf - dbhome_1 | gzip > dbhome_1-20150211.tgz
cp -r oraInventory oraInventory.20150211
But the production cluster has 12 nodes (more...)
I saw a load of 44 on a node of our production Exadata and it worried me. The AWR report looks like this:
CPUs Begin End %User %System %WIO %Idle
----- --------- --------- --------- --------- --------- ---------
16 10.66 44.73 68.3 4.3 0.0 26.8
So, why is the load average 44 and yet the CPU is 26% idle?
I started looking at ASH data and (more...)