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Can you add a SATA drive in an existing SAS Raid-0 array

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January 29, 2010 1:38:04 PM

Hi Folks,

PERC5/i controller. Can I add a SATA Drive in an existing Raid0 array. The array has 3 SAS drives.

Thanks
a c 126 G Storage
January 29, 2010 1:58:12 PM

Add a drive to an existing array; meaning you get a 3-disk RAID0 array? You can do that with any RAID controller:

1) delete the current RAID
2) re-create the RAID with 3 drives

You will, ofcourse, lose all data currently on the RAID.
January 29, 2010 6:07:44 PM

Thanks for the reply,

I've already got a 3 SAS disk array as I said before. And I am aware that I can recreate the array loosing the data. My query was if I could Add a SATA drive to the existing array?
I take it from your answer that you think that's not possible without recreting the array! Any thoughts on mixing SAS and SATA drives?

Cheers
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a c 126 G Storage
January 29, 2010 6:23:31 PM

In theory you could add a drive to a RAID0 array just like some hardware controllers allow you to 'expand' a RAID5 array with one or more additional disks. But that's theory, and this is not implemented in any drivers or hardware to my knowing. So the answer is: no you can't. :) 

You should be able to mix SAS and SATA, because SATA disks can still be connected to a SAS (RAID) controller. Just not the other way around: connecting a SAS disk to a normal SATA controller won't work. It may also be that the specific controller you use doesn't allow SATA disks because of bugs or by design; but there's no real reason it shouldn't work.

Of course, it would mean you add different harddrives with different performance characteristics in the same striping array; where the slowest disk may affect the performance of other disks as well.

Also, with RAID0 (striping) and RAID4/5/6 you need to be aware of stripe misalignment issues. For example, Windows by default creates a partition on a RAID0 array which starts at 31.5KiB or 63 sectors. Ideally this should either be 0 (then it won't be bootable; unsupported by windows) or a larger value like 128KiB or 1024KiB. Vista and Windows 7 are capable of making partitions starting at 1024KiB. This will prevent both disks serving the same I/O request; as ideally they should both be working on separate I/O requests so you get 2 I/O's done in roughly the same time you normally can do only 1. For this to work, you need an aligned partition (aligned to the stripesize blocks) and a stripesize of at least 128KiB and preferably larger.

So you see, some tuning is often needed for optimal performance. Wish you the best of luck and hope any of this helps.
a b G Storage
January 29, 2010 6:30:53 PM

Adding (or removing) any drive from a RAID-0 array will cause loss of data due to the way the Array works.
Most SAS controllers will happily use (and mix) SATA and SAS drives, However mixing the two will slow the SAS drives to the speed of the SATA drive and waste a lot of (potential) speed.
February 1, 2010 8:42:20 AM

My information is the opposite.

I can see where the confusion might come from... SAS controllers can perfectly happily read SATA drives.

However, they will not usually allow you to create one array that has both types of drive.

You can create one array of SATA drives alongside another array of SAS drives, but not one array that has both. This is the case in my LSI 1068 SAS RAID board. (Asus Z8PE-D12X.) When I begin to create an array, all disks are shown. If I select a SATA disk, the SAS disks are marked incompatible and cannot be chosen, only other SATA disks.

There may be a manufacturer that has boards that do allow mixed drives but I still would not recommend it. Every shred of RAID guide on the internet recommends identical drives from spin speed to cache size and sometimes to manufacturer, though those are being paranoid and there may be good reasons that you would WANT to mix the manufacturers. (To avoid statistical clustering of drive failures towards the end of their life.)

Changing the whole connective protocol seems like a more drastic difference even than cache size. Besides, if you have SAS disks my guess is you chose to spend that extra cash so you could get 15k rpm spindles... Mixing the 15k spindles with 10k or 7200 also goes against most of the advice I've read.


Side note, slightly OT but I'm leaving it here for the benefit of people who land on this thread while debating moving into SAS:

Having broken into SAS myself, I now wish I had stuck with SATA Raptors instead. The cost overhead has not led to really significant improvement, and I am in a field that would see that improvement on a daily basis. (uncompressed HD multi-layer video compositing, hundreds of MB per second dynamics caches...)

Plan on extra expenses far above cost of drives. You will have to use hot-swap bays because the drives do not connect directly. You may need a new case because of the length of said hot swap bays. Hot swap bays need their built-in fan and it is not a quiet one. You will have to spend at least $300 on the RAID card alone to get the array type you want that is available with onboard SATA. (Raid 10. Use it. You'll see.) That includes the Asus PIKE cards, which after owning one I can say have limitations. (two array limit and lack of raid-10 below that price point, for one.) I know there are adapter cables to plug drives in directly. Don't use them. They do not fit tightly and will pop out of the drive when other cables move around or even from their own cable stiffness. I lightly touched the connector once while troubleshooting constant BSODs and even though the cable was "secure" the light contact was enough to kill the connection, reset the OS drive and BSOD the computer.

Cost Overhead for SAS: Adapter Cables ($20-$40 each), increased controller cost ($350 or so), hot swap bay ($200 or more unless you want something super cheap-o), drives ($50 to $100 difference, going roughly off memory.)

For the cost overhead of SAS you could get additional raptors for more benefit. Or a pair of SSDs.

SAS is for when you have a huge rackmount server with 16 to 32 drives, and think you might add more.
April 16, 2011 10:06:38 PM

I don't think that your array would be stable or at best it would slow down to SATA speed instead of SAS speed. The whole purpose of SAS is dependable arrays
throwing in a SATA will negate any good from having SAS drives. If you want
an inexpensive array build on SATA and be done if you need speed, reliability and size
then bild a SAS.
!