RAID

Los Angeles1
Los Angeles1 used Ask the Experts™
on
What would be a good model of RAID for the following scenario.  

On an 8 core Intel based Linux machine (yet to be determined) that will run WMQ and perform lots of sequential logs, we are looking for a mutli drive RAID with lots of disk cache to be used in write back mode, and will operate in RAID 10.  To idea is to not be overly slowed down during peak times when data is written to the logs.  The RAID 10 can not be avoided.

What specific models of RAID would be good candidates?
Comment
Watch Question

Do more with

Expert Office
EXPERT OFFICE® is a registered trademark of EXPERTS EXCHANGE®
DavidPresident
Top Expert 2010

Commented:
RAID10 is not appropriate for log files.  You want RAID1. Why? Because the log files are short record lengths, and RAID10 performance is geared towards high throughput sequential reads and writes.

RAID1 is better for transactional R/W data, like in log files.

Commented:
RAID 10 is the best choice. RAID 10 is used for systems which require high write performance.


RAID 10 is classified as the futuristic RAID controller with extremely high Reliability and performance embedded in a single RAID controller. The minimum requirement to form a RAID level 10 controller is 4 data disks. The implementation of RAID 10 is based on a striped array of RAID 1 array segments, with almost the same fault tolerance level as RAID 1. RAID 10 controllers and arrays are suitable for uncompromising availability and extremely high throughput required systems and environment.
DavidPresident
Top Expert 2010

Commented:
First RAID10 can be accomplished with 1-4 disks.  Pretty much every UNIX software-raid will let you build a RAID10 config if you partition it properly with 1-4 disks.  Heck, even windows doesn't require 4 disks for RAID10.

As far as RAID10 being "best".  This isn't always true, and rarely is true if you just have 4 disks to work with.   If you properly configure everything then you can make 2 x RAID1 arrays outperform 1 x RAID10 in both IOPs and throughput.  But I admit, most people don't know how to do this.

Databases are NOT all about throughput.  They are TRANSACTIONAL in nature.  Index files are subject to random I/O.   Depending on the implementation, a RAID10 might require 64KB reads and/or writes.  But RAID1 can be tuned to 4KB I/Os.  So if you only have to write 4 KB worth of data, your RAID10 will force I/Os to be in multiples of 64KB.

Worse, a more likely scenario,  you have to do a dozen 4KB I/Os in an index file to get to 64KB data record.  With a RAID10, you are going to be tying up all 4 disks doing 12 x 64KB I/Os, then you get the 64KB worth of data.  But in 2 x RAID1, you do 12 x 4KB I/Os on 2 disks, then if data is on the other disk you do a single 64KB I/O.

Where it really gets better is when there are multiple I/O requests.  The two RAID1 sets operate independently and you can set more appropriate block sizes so your disks only read/write what is required by the O/S and application software.   You don't have that luxury with RAID10.

Author

Commented:
dlethe

The scenario that I am on is a sequential log file (actually 3 to 16 log files) all written to seuentially, with no index files.  

In such a case would RAID 1 or RAID 10 be better?

In the scenario above, if the log files are written to sequentially, but on a transaction basis (again no index files), which would be better?
President
Top Expert 2010
Commented:
With 3-16 log files written simultaneously then you aren't doing sequential I/O as much as mixed I/O.

Every time you write to a log file you have to update the headers, and if your code does

open -> append -> close then you are doing a great deal of random I/O.   That being the case, RAID1 is generally better.

But if it is open -> write -> write ....   and then you close after writing a large amount of data, and all the files generally stay open, RAID10.

So what does the code do?   If the files stay open, then how many bytes are written via your source code write statement before your app writes to a different file?

Do more with

Expert Office
Submit tech questions to Ask the Experts™ at any time to receive solutions, advice, and new ideas from leading industry professionals.

Start 7-Day Free Trial