Promise RAID-1 vs software RAID-1

[Lucas Harms]

I am fairly certain Linux software raid 1 does consider one of the drives as
the master drive.  Instructions to restore damaged arrays will indicate
that.  You can switch the drives around or move the slave to the master, but
it still makes a distinction.

Since a raid 1 is only about security, I doubt a proper raid 1 setup would
allow you to read from both drives.  There is the possibility that through
some bug, a write and read could occur at the same time if the drives are on
different ide controllers.

I've run several Promise raid 1 arrays on most of the cards up through the
SuperTrack series (which is an actual hardware raid card).  I've never seen
any sort of performance boost when compared to software raids or even
regular ide/ata performance.  But that is just me.

Luke Harms
lharms at scenpro.com

-----Original Message-----
From: fedora-list-bounces at redhat.com [mailto:fedora-list-bounces at redhat.com]
On Behalf Of Rodolfo J. Paiz
Sent: Wednesday, June 16, 2004 12:06 PM
To: For users of Fedora Core releases
Subject: Re: Promise RAID-1 vs software RAID-1

At 10:39 6/16/2004, Alan Horn wrote:
>Raid 1 is mirroring only. For data integrity reasons I can't see how they 
>could read from any but the master mirror.

WAG alert:

Once the two disks are synchronized, it sounds perfectly reasonable to 
writes to take the same amount of time (since all data has to be written to 
both drives), but for reads to be significantly faster by reading from both 
spindles. There really is no "master" in an equal set of two, I think. Both 
contain identical data, so you should be able to read from both. I think.

By the way, the Promise cards *are* software RAID... all the real RAID 
computations are done by the driver using the computer's resources. The 
trick is that the binary driver for Linux is probably less mature and less 
flexible than Linux native software RAID, so I've always used Promise and 
HighPoint controllers simply as additional EIDE/ATA controllers and used 
Linux software RAID. Excellent results. Note that Linux software RAID is 
also capable of RAID-5, IIRC, which the Promise drivers are not.

If you have the budget, I'd strongly recommend the 3Ware cards. Real 
hardware RAID, native Linux kernel support since 2.2.x, fast, very 
reliable, and not expensive.

Cheers,


-- 
Rodolfo J. Paiz
rpaiz at simpaticus.com
http://www.simpaticus.com


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