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Author Topic: Raid using external USB drives  (Read 18193 times)
BlackeyCole
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« on: January 26, 2007, 12:46:07 PM »
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I am interested in creating a Raid system using several USB external drives.  I use a Win XP Pro OS.  I know there is a way to create a raid using disk management of the XP admin tools.  Is this the best way or is there a USB device that acts as a hub and create the raid using the drivce conected to it or is there software to create the raid using conected drives.  I want to use the RAID that uses three drives to stripe the information on and a fourth to contain the checksum info so if one drive fail you can replace it and recreate the data so nothing is lost.  My goal is to use 40 or 60 GB external drive conectted to a hub so everything is coneected at once  This would make it hot Swappable since  USB drives can be added and removed without  rebooting. And you can mirror or backup that to another drive that is as large or larger than the stripe drive and that way you can recreate lost information and you have a backup of all the info if you have a multi drive failure at once.

I know you can buy a raid enclouse that connects through the usb or firewire ports but my idea is to create one using drives that I already have.
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feppe
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« Reply #1 on: January 26, 2007, 05:15:00 PM »
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I would strongly advice against this. If you're planning to build a RAID for security and redundancy, this is not the way to do it.

I hope people with more experience with RAIDs can set you straight, but I believe software-based RAID setups are more prone to errors than hardware. The drives have to be the same exact size.
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John.Murray
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« Reply #2 on: January 26, 2007, 06:37:23 PM »
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There is a *lot* of mis-information about raids out there!

Myth:  After replacing a failed disk, the RAID array, will automatically re-build

Nothing could be further from the truth!  A failed array must be manually re-built by the system administrator - the process once begun is largely automatic.  Why the human intervention?  Lets say someone wants to steal my data - if the array automatically rebuilt itself, all they would need to do is walk in with 3 drives and some time . . .  There are RAID solutions involving a 4th "hot spare"  - typically higher end SCSI controllers with battery backed up cache . .

Did you realize that ATA or SATA drives actually have the "controller" built on the drive?  The ATA and SATA cards or built in chipset on the mainboard are nothing more than BUS controllers!  This make RAID effectively software driven as all "intelligence" is actually performed at the O/S device driver level.  Contrary to opinion Windows RAID is actually pretty good - an advantage of it is that it is instrumented, meaning that any problems or disk failures will be reflected in the Windows system event log.  If you are considering an ATA or SATA solution, make sure it supports WMI, the better Adaptecs do.

A USB RAID is a *bad* idea, forget it

Consider an external NAS solution - Advantages are complete independance from workstation O/S, ability to share access to multiple machines.  Disadvantages are access speed -  currently limited to GB-Ethernet througput, a number of users are using several vendor's NAS solutions - I'm currently building my own based on FreeNAS ( http://freenas.org )

Don't consider RAID an alternative to backups

Finally, if you choose a RAID solution, be *sure* you know how to recover a failed disk, and monitor ongoing health, I've seen too many users initialize their array after replacing a drive

hope this helps -John
« Last Edit: January 26, 2007, 06:44:22 PM by Joh.Murray » Logged

Olli Vainio
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« Reply #3 on: January 26, 2007, 11:31:03 PM »
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I hope people with more experience with RAIDs can set you straight, but I believe software-based RAID setups are more prone to errors than hardware. The drives have to be the same exact size.
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I've found Linux's software RAID to be about as reliable as hardware solutions, both have had errors but nothing substantial.

And like John also said, but I will say it again: RAID's should NEVER, EVER be used as a replacement to backups.
(My photos are on my external FW drive and on 2 separate file servers(both have RAID5 HDDs). And other one of those file servers is on completely different location than the other.)
« Last Edit: January 26, 2007, 11:31:32 PM by Olli Vainio » Logged
Jonathan Wienke
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« Reply #4 on: January 27, 2007, 01:42:55 AM »
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What you're asking for is a bad idea, for all the reasons mentioned in the previous post, as well as these:

It's better to have all of your RAID drives in a secure enclosure. If a USB cable accidentally gets yanked from a single drive enclosure, no big deal, reconnect and drive on. If this happens to a drive in an external RAID array,, now you're operating in failure mode and the controller is going to need to do a rebuild after you reconnect. If whatever disconnected the single cable happens to disconnect two, you just lost all of your array data unnecessarily. If you have a cat or small children who have access to your computer area, this is not paranoia, it probably will happen at some point unless you were to make your enclosure to secure the array together and prevent the cables from accidental unplugging.
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nemophoto
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« Reply #5 on: January 31, 2007, 01:51:11 PM »
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Blackey,

As has been noted, USB is not the RAID drive of choice. If you want an external solution, go with a RAID setup using eSATA drives and housing. USB was never really intended for or designed for RAID usage, though it's obviously been pressed into service.
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jani
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« Reply #6 on: January 31, 2007, 05:17:12 PM »
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There is a *lot* of mis-information about raids out there!

Myth:  After replacing a failed disk, the RAID array, will automatically re-build

Nothing could be further from the truth!  A failed array must be manually re-built by the system administrator - the process once begun is largely automatic.  Why the human intervention?  Lets say someone wants to steal my data - if the array automatically rebuilt itself, all they would need to do is walk in with 3 drives and some time . . .  There are RAID solutions involving a 4th "hot spare"  - typically higher end SCSI controllers with battery backed up cache . .
I don't think countering misinformation with misinformation is a good idea.

A fair amount of RAID controllers have a feature called "auto rebuild", which is used to automatically rebuild arrays.

This is a perfectly normal feature in controllers supporting RAID 5, almost mandatory.

Cheaper and/or older controllers require a hot spare for rebuilding the array, more advanced and/or recent controllers can perform a bit more advanced action:

Quote
The Auto Rebuild policy determines how the controller firmware will attempt
to rebuild degraded units.

When Auto Rebuild is disabled, only spares will be automatically used to
rebuild degraded units. When Auto Rebuild is enabled, the firmware will
select drives to use for automatically rebuilding a degraded unit using the
following priority order.

Smallest usable spare.
Smallest usable unconfigured (available) drive.
Smallest usable failed drive.

Enabling Auto Rebuild allows you to add a drive to the controller and have it
be available for a rebuild, without having to specify it as a spare.

With Auto Rebuild enabled, if you accidentally disconnect a drive (causing
the controller to see it as a failed drive) and then reconnect it, the controller
will automatically try to use it again.

You can enable or disable the Auto-Rebuild policy through 3DM or 3BM.
You will note that the 3ware controllers are not SCSI controllers, but SATA and PATA controllers.

And if someone has physical access to your drives in the first place, you're screwed in terms of data security, unless the volume or data is securely encrypted, regardless of whether it's possible to rebuild a RAID or not; RAID auto-rebuild or not is mostly irrelevant for that problem area.
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Jan
John.Murray
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« Reply #7 on: January 31, 2007, 10:59:29 PM »
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Thanks Jani!  I stand corrected - my comments are based on SCSI based PERC controllers.

Regards - john
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jani
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« Reply #8 on: February 01, 2007, 01:13:34 AM »
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Thanks Jani!  I stand corrected - my comments are based on SCSI based PERC controllers.
PERC, as in the Dell product?

Oh, boy ... That is practically a swearword.

If you ever should need to comment on "blade technology", make sure that you study products and technology from other manufacturers.
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Jan
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