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Old 3rd February 2007
  #1
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Question RAID

Rather than hijack the sticky, I'll start another thread.

I have an ATTO UL3D SCSI card. As the D in the model name might suggest, it is a dual-channel card. On one (1) of these channels, I have a RAID 0 - eight (8) Seagate 36GB drives striped as a single disk. I have no data security here, only performance.

I am moving into applications where I need more bandwidth. Though within theoretical limits, the UL3D is choking on what I will eventually throw at it more often. I have spoken with the company that makes the hardware I use in this application, and they suggested the UL4D (Ultra 320, as opposed to Ultra160). They also suggested that I build another tower of eight (8) drives, giving me a total of 16, and stripe them all as one disk.

So the questions are - can the 16 drives be striped as one at once? Do I have to stripe two (2) groups of eight (8) drives as RAID 0 first (since each group of eight drives is on a different SCSI channel)? If I have to create two (2) RAID 0 arrays first, do I then take those two (2) RAID 0 arrays and then make them a RAID 5 to create the one massive disk?

Thanks for reading!
Old 3rd February 2007
  #2
Quote:
Originally Posted by hociman View Post
Rather than hijack the sticky, I'll start another thread.

I have an ATTO UL3D SCSI card. As the D in the model name might suggest, it is a dual-channel card. On one (1) of these channels, I have a RAID 0 - eight (8) Seagate 36GB drives striped as a single disk. I have no data security here, only performance.

I am moving into applications where I need more bandwidth. Though within theoretical limits, the UL3D is choking on what I will eventually throw at it more often. I have spoken with the company that makes the hardware I use in this application, and they suggested the UL4D (Ultra 320, as opposed to Ultra160). They also suggested that I build another tower of eight (8) drives, giving me a total of 16, and stripe them all as one disk.

So the questions are - can the 16 drives be striped as one at once? Do I have to stripe two (2) groups of eight (8) drives as RAID 0 first (since each group of eight drives is on a different SCSI channel)? If I have to create two (2) RAID 0 arrays first, do I then take those two (2) RAID 0 arrays and then make them a RAID 5 to create the one massive disk?

Thanks for reading!
Hi Jon...

The amount of Drives that can be added to a strip set is thoretically not limited with the exceptions of limits based on implementation of the hardware and software/firmware by any given manufacturer of RAID sub-systems. ( that was helpful huh... ) The real question is if you have 2 physical drives on a strip set creating a single drive you have created a faster logical drive that has twice the potential for data loss. As you add drives to a logical drive set with no redundancy you continue to increase the potential for failure while not providing a linear potential for speed. ( Yes, speed performace does go up assumig the driver implementation actually uses the ability to parallel process properly. ) Additionally the slowest part of a data transfer from disk to memory is the seek time ( spinning of the disk until the data is positioned under the heads ) and latency ( the disk head arm moving back and forth actually reading the data from various sectors ). So, if you create larger directories ( ie: more files on a given disk ) you create more seek/latency time to locate the data and retrieve it. As you add more/remove and create files you add the potential of more disk fragmentation, thus increasing the total number of disk reads, adding more time to a transfer.
Anyway, doubling the number of drives, and thus doubling the number of heads does not double the performace... as you start to add bunches of drives I personnaly feel ( having run *HUGE* data centers in past I.T.lives ) that adding more than 8 drives to a strip set is starting to waste money and asking for a data hit. I'd create multipe strip sets and mirror them. Generally, with high performace RAID controllers you have a significant cache allowing for the dual writes to occur off-line keeping write performace on par with a single strip set and allowing read performace to increase via the controller determining which mirror set of a given drive is closest to the data on the disk. you can do this two ways:
1. create a mirror of each disk and then strip the disks
2. create a strip set and the mirror the set
Performace will vary with both methodologies, tests would be the best way to determine which is right for the app. I would also suggent speaking with the tech team from the RAID controller company to discuss the options. the real question is, are you going for write perfromace or read performance and what type of data are you transfering...
I'm in the office today if you want to call me to chat...

Cheers
geo
Old 3rd February 2007
  #3
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Lightbulb stripe it

Hey Geo,

So your response, coupled with Google, has got me going in the right direction. I have a degree of confirmation from reputable sources that the UL4D is worthy. I'll ask another question that you might know the answer to. Instead of 8 drives per channel, I might use 4 drives per channel. Can Disk Utility be used to stripe the four drives on each channel into one usable (logical?) disk?

Thanks for the response!
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