I’m not sure how this will go down, but I’m about to disagree slightly with my homeboy blogger JoeD regarding SAS’s effect on other storage options. It’s not that he’s necessarily wrong, but I’m going to present a different answer that comes from a different point-of-view.

First, Joe said that SAS is mostly a replacement for parallel SCSI. Of course he is 100% correct.

But it’s important to realize that SAS also replaces ATA. Before you jump up and say, “But SATA drives are so much cheaper!”, let me remind you that SAS isn’t just a drive interface. It’s also an adapter interface. It’s the interface on your motherboard or on your RAID controller. So if you were going to move from ATA to some new interface, you might not select SATA. That would lock you into SATA drives. But selecting SAS will allow you to use either SAS or SATA drives, as I’ve mentioned before. So while most people won’t be able to justify more expensive SAS drives, they will at least have the option to use SAS drives.

Next, let’s look at FC. It has two purposes – as a fabric and as a drive interface. I’m not going to touch the fabric purpose. Unless you squint and hold your mouth just right, SAS has no real play there. But as a drive interface, let’s look at what FC drives are being attached to.

If you own a FC-to-FC MFA (Mid Fabric Appliance) or RAID controller and you like to hang lots and lots of FC JBODs behind it, or put those JBODs really far away, then you obviously have to keep buying FC drives. But if you buy a FC-to-FC MFA or RAID controller, then you should be asking your vendor to make a FC-to-SAS version. SAS has all the reliability and performance of FC, and you also have the option of using cheaper and larger SATA drives.

You may be using FC drives simply because you have an external JBOD with drives outside your server, and you might not like nasty parallel SCSI cables. This is a typical “direct attach” model. Or you may have multiple hosts connected to that storage for failover purposes, and of course parallel SCSI cables is a nightmare in this configuration. This is obviously the ideal usage model for SAS. Anybody using FC in such small systems would be crazy to not switch to SAS. And like I’ve mentioned before, you will have the option of using cheaper SATA drives.

Lastly, believe it or not, SAS will encroach on some number of iSCSI sales. Like the FC SAN example, I’m going to ignore all the larger, more complicated fabrics where iSCSI makes sense. But there are companies that sell iSCSI as a direct attach solution. And originally this really didn’t seem that stupid of an idea - it has better cabling than parallel SCSI, it’s not too hard to manage, etc. But now with SAS, that idea actually is kind of stupid. SAS has much higher performance, is much cheaper, and, again, it supports SATA.

Let’s summarize:

SAS replaces parallel SCSI.
SAS host adapters replace ATA.
SAS supports SATA drives.
SAS replaces direct-attach FC.
SAS replaces FC in external RAID systems.
SAS replaces direct-attach iSCSI.
FC is used in high-end SANs.
iSCSI is used in low-end SANs.

Hmm, I wonder what happens when 10Gb iSCSI is released? Where does FC go? Well, I don’t pity the FC vendors. They dug their own grave by selling two port FC adapters for $1000 and FC switches for $500 per port. SAS and iSCSI may have never happened if they hadn’t been so greedy.

But I got off topic. SAS will take major market share from FC, some from ATA/SATA, and even share from iSCSI.

TT

This entry was posted on Friday, October 28th, 2005 and is filed under Storage Interconnects & RAID, Advisor - Tom Treadway. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.