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Thinking about setting-up as distributed RAID array over iSCSI. Has anyone else tried this, if so what was your experience.

To be more specific I am thinking of a couple of CentOS servers each with say four drives, all published as iSCSI targets. Then one Windows Server accessing all of these via iSCSI and connecting those targets together to create a software RAID array.

One of my concerns is the rebuild time if one of those servers is offline for a short amount of time. Would it then be necessary to rebuild the entire array from scratch or is software RAID clever enough to only rewrite sectors that have changed? My concern being that a small network glitch could otherwise potentially cause a long rebuild process.

Thanks, Nick

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Why would you want to do this instead of creating RAID on the server and export this as one iSCSI volume? This sounds like a pretty delicate environment. –  Sven Nov 13 '12 at 23:26
What problem are you trying to solve with this? –  MDMarra Nov 13 '12 at 23:40
Are you wanting a highly available iSCSI target? that actually makes sense. –  Matt Nov 13 '12 at 23:55
@Matt: HA is about eliminating possible points of failure, not creating them. –  Sven Nov 14 '12 at 0:35
If you want a distributed filesystem, build a distributed filesystem. There are plenty out there; some even work under Windows. –  mfinni Nov 14 '12 at 1:32

2 Answers 2

up vote 1 down vote accepted

This has many drawbacks and not a single advantage I can see, so I don't understand why you would want to do this.

  • Any outage of the iSCSI connection will likely require a full RAID rebuild. The RAID subsystem doesn't know that the disc is the same if it sees it again and that it is more or less unaffected, but even if that would be the case, it doesn't keep a log of write operations since the failure that it could use to bring the drive up to date again.

  • The network connection will be a serious bottleneck, especially in case of a rebuild. You will have a small number (likely just one) 1 GBit/s network connections compared to multiple SATA/SAS connections with up to 6 GBit/s each, connected over the PCIe bus.

  • This whole setup is really delicate and easy to bring to a complete halt.

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The whole idea is to create a robust distributed RAID environment using iSCSI targets so that there is not a singular point of failure. By the sound of the comments here iSCSI might not be the best way to do this. To me the advantage of iSCSI is that it lets me use Linux servers to store NTFS based data for our Windows users. –  NickC Nov 14 '12 at 15:51
This can be done via iSCSI, but doing it reliable will involve a lot more than your somewhat simplistic approach. If it needs good performance, it will be expensive as well (think multiple 10 GB ethernet connections/switches or even Infiniband to reduce latency). Other approaches might be more appropriate and cheaper to reach that goal. –  Sven Nov 14 '12 at 16:07

I wouldn't do it this way. There are lots of manufactured NAS devices with multiple disk arrays both for home use and business use that have ISCSI built in. These devices are designed with this purpose in mind and will outrun the software configuration that you are thinking about at a comparable or cheaper cost.

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