RAID 5, plus HDD or RAID 0 for system?

[Ubtree]

I intending to build a RAID 5 workstation, and am trying to get to grips with a couple of RAID issues.

1. I understand the need, from a security point of view (in case the boot sector is corrupted), to keep the Windows system separate, but I thought that it was sufficient to keep it on a separate partition. I have read on this forum that for good RAID performance you need to have a system HDD in addition to the RAID array. Why is this?

2. Having the system on a RAID 1 pair would presumable be preferable (resilience + improved read performance) to having it on a single drive. I have read that the performance of an onboard RAID controller is comparable to the performance of a RAID controller card for RAID 0 and RAID 1. So would it make sense to have the system on 2 disks controlled by the motherboard in a RAID 1 array (for the system), and 4 data disks controlled by a RAID controller card in a RAID 5 array?

Any help in answering these questions would be greatly appreciated.

[uOpt]

1. I understand the need, from a security point of view (in case the boot sector is corrupted), to keep the Windows system separate, but I thought that it was sufficient to keep it on a separate partition.


Glibberish, nonsense.


I have read on this forum that for good RAID performance you need to have a system HDD in addition to the RAID array. Why is this?



It isn't.



2. Having the system on a RAID 1 pair would presumable be preferable (resilience + improved read performance) to having it on a single drive.



The onboard sata raid junk generally doesn't give you any speedup out of RAID-1.

If you have good raid-1 you get improved small reads after random seeks, but other patterns such as contiguous reads and writes are not sped up at all no matter how good your raid software is.

Raid-5 does give a speedup on most read patterns.



I have read that the performance of an onboard RAID controller is comparable to the performance of a RAID controller card for RAID 0 and RAID 1.


No, and the onboard sata raid stuff gets beaten by real software raid easily, too. Which also does raid-5 very well.

What you say above was true in the timeframe of Pentium Pros. With 2+ GHz AMD64s and the current lame harddrive the main CPU can easily do raid-5 xoring for one or two 3-5 disk raid-5s with CPU to spare.

The reason why onboard sata raid often doesn't perform as well as it should is lousy software.


So would it make sense to have the system on 2 disks controlled by the motherboard in a RAID 1 array (for the system), and 4 data disks controlled by a RAID controller card in a RAID 5 array?


No.

I wouldn't trust any of that onboard sata stuff for anything but raid-0 anyway (although I didn't test Intel's version).

If you do trust it, then the above theories that lead you to split you into two arrays are all invalid.

[protias]

I wouldn't trust any of that onboard sata stuff for anything but raid-0 anyway (although I didn't test Intel's version).

If you do trust it, then the above theories that lead you to split you into two arrays are all invalid.

That's about all I agree with your post. The rest of it is pretty much garbage. :rolleyes:

[unhappy_mage]

1. I understand the need, from a security point of view (in case the boot sector is corrupted), to keep the Windows system separate, but I thought that it was sufficient to keep it on a separate partition. I have read on this forum that for good RAID performance you need to have a system HDD in addition to the RAID array. Why is this?
I like having a separate boot disk so that the array going down won't mean I can't boot the machine. If I were to have disk problems and not be able to load the array, at least I could use the OS to help debug things. In terms of performance... well, if you're swapping on raid 5 you'll wish you were dead. On a single disk that'd be a good deal faster.
It isn't.
Perhaps not for speed, but as I said, I like having the OS separate for other reasons.
The reason why onboard sata raid often doesn't perform as well as it should is lousy software.
Agreed; and thus hardware raid has made its name.

[Madwand]

1. I understand the need, from a security point of view (in case the boot sector is corrupted), to keep the Windows system separate, but I thought that it was sufficient to keep it on a separate partition. I have read on this forum that for good RAID performance you need to have a system HDD in addition to the RAID array. Why is this?

When you're doing RAID construction or maintenance, you'll often find that it's more convenient to have the OS separate from your data. Especially when you get into feature-limited motherboard RAID and large data sets. As for performance, it could go both ways -- by having another spindle attached to the RAID array, you could get somewhat better sequential access performance. Conversely, by having some data (the OS, swap, personal folders, etc.) on a separate drive from the array, you could get better concurrent access performance sometimes. For small random write access patterns, anything would likely be better than RAID 5.

Having a separate partition for the OS on one big array would be good from a data segmentation point of view (the OS, drives, boot image, etc., would not be on the same partition on your data, hence your data partition would be easier to back up and maintain). However the array underlying the OS would still be a complex RAID array, and when it came time to maintain or rebuild the array, you'd have to deal with the effects of the OS.


2. Having the system on a RAID 1 pair would presumable be preferable (resilience + improved read performance) to having it on a single drive. I have read that the performance of an onboard RAID controller is comparable to the performance of a RAID controller card for RAID 0 and RAID 1. So would it make sense to have the system on 2 disks controlled by the motherboard in a RAID 1 array (for the system), and 4 data disks controlled by a RAID controller card in a RAID 5 array?

I wouldn't count on the RAID 1 read performance improvement with most RAID implementations, but otherwise it is reasonable to have the OS and data on separate arrays. The OS array should not need as much maintenance as the main data array, and being able to separately maintain the main data array would be better. Using RAID 1 would be an additional boon, because RAID 1 can often be treated as simple single drives. E.g. if you had a problem with the motherboard, you could conceivably take the set to a different motherboard, have the OS array recognized as a single drive, and later extend it to a new RAID 1 array. The data array would be a separate problem.

Finally, most of the details and features and performance depend greatly on the particular RAID implementation (and the needs of your intended application). You should supply the details of the prospective motherboard, array size, candidate RAID implementations, application, and backup plans and needs, to potentially get more useful advice in context.

[Ubtree]

Glibberish, nonsense.



It isn't.



The onboard sata raid junk generally doesn't give you any speedup out of RAID-1.
.
Thank you for your comments, uOpt. Unfortunately they do not help me, nor I suspect will they inform others who hope to learn something from this thread.

protias, unhappy_mage and Madwand, thank you for your advice, which is very helpful. It will help me in drawing up my system design. I’ll come back for further comments, as Madwand suggests, when I have come up with a firm proposal.