Random and Sequential Read/Write !!

[Chris Fung]

Dear all,

I have a question about performance of hard disks !! Can any tell me the difference between Random and Sequential Read/Write in terms of hard disk access ? Any examples for what kinds of applications will use random and which kinds will use sequential ?? What is the different between Unix file copy and database access?? Which kind of read/write they belongs to??

Appreciated if anyone can help answer my questions with any resource links.

Many thanks,

Chris,

[steven Burgess_2]

Hi Chris

Have a look at the below


http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/92431-90014/92431-90014_top.html&con=/hpux/onlinedocs/92431-90014/00/00/20-con.html&toc=/hpux/onlinedocs/92431-90014/00/00/20-toc.html&searchterms=read%7cwrite%7csequential&queryid=20020706-000018



http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/mpeix/onlinedocs/32650-90885/32650-90885_top.html&con=/mpeix/onlinedocs/32650-90885/00/00/47-con.html&toc=/mpeix/onlinedocs/32650-90885/00/00/47-toc.html&searchterms=read%7cwrite%7csequential&queryid=20020706-000018


Hope this helps

Steve

[Steven Sim Kok Leong]

Hi,

For Oracle database, index and database filesystems are striped across disks (and preferably across controllers as well) because reads and writes of data are random. Whereas the database redo log filesystems should not be striped because they are always sequentially written into and sequentially read when needed.

You can use free tools such as stkio (stkio.hp) from Storagetek to measure the throughput and response time of random and sequential reads and writes of varying size files with varying block sizes. The last time I checked, it was downloadable from the storagetek website.

Hope this helps. Regards.

Steven Sim Kok Leong

[Bill Hassell]

In a very simple computer system with one user and one disk, then it might be possible to characterize random and sequential access. Random access is not very common when you are writing email or using a browser. Sequential is fairly common for small tasks.

However, add a few users doing different tasks, throw in a database with many more users accessing the data, and all at the same time, the disk(s) will be mostly randomly accessed. As mentioned, there are ways to mitigate the head movements (thrashing) by moving sequential across multile disks and disk channels.