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superloopy1

External parity drive?

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Tell me if this sounds ridiculous but i'm becoming paranoid about only having single parity against my array of 4x8TBs. Unfortunately i have no spare slots available and i need to migrate stuff off the array if i'm ever going to go dual parity. But, moving stuff with just one parity drive seems like an accident waiting to happen as far as i'm concerned.

 

So ... is it possible to hook anything up externally which will 'act' as second parity and will then move into the array once i've achieved the moves needed. I've plenty of 8TB drives just no physical slot within the array.

 

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24 minutes ago, superloopy1 said:

So ... is it possible to hook anything up externally which will 'act' as second parity and will then move into the array once i've achieved the moves needed. I've plenty of 8TB drives just no physical slot within the arra

esata or USB. 

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5 minutes ago, superloopy1 said:

Tell me if this sounds ridiculous

it does :)

 

What do you mean exactly by 'slot'? I usually say 'bays' when talking about physical locations in a case, and 'ports' when talking about drive connections. 'Slots' I reserve for the actual unRAID disk assignments, such as disk1, etc. If you have ever read syslog you will know that unRAID uses the word 'slot' for this purpose, parity is slot 0 (and parity2 is slot 29).

 

If you have a free SATA port, you could run an eSATA connector from it to an external enclosure.

 

But with the small number of drives you have dual parity isn't the first thing I would be considering. By far the main reason people post here with a disabled disk is due to bad connections, with controller issues a distant second place and actual bad disks fairly rare. And the reason they get a second failure when trying to fix the first is also bad connections, by disturbing other connections while replacing a disk. Good cables, backplanes, drive cages and above all, just being careful, are some of the things you can do to prevent these occurrences.

 

Another reason people wind up with multiple disk problems is because they are unaware they have a problem until they get another. unRAID with single parity will emulate a disabled disk, and let you continue to use the array, and even read/write, to the disabled, emulated disk. Then some time later, they get another disk with a problem and it no longer works. The best way to prevent this is to setup Notifications, so unRAID can tell you immediately when the first problem occurs.

 

And even when multiple problems do occur, often little if any data loss will result if you follow good advice, assuming you don't have multiple disks that are totally unresponsive.

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If you have one drive that is mostly static with no new content being created on it, and accessed very little, this is the drive I would make external.  The second parity needs to have fast reliable access, and the external may not have that, depending how you hook it up.

 

However, I think you are being overly paranoid.  The external drive is very likely to cause you more problems than it will solve. 

 

Time for an upgraded chassis if you are serious about needing dual parity.  And get one with hot-swap drive capability.  I can't count the number of times that I have bumped a cable and caused the array to burp a drive offline during routine maintenance.  Since I went to 24 bay SuperMicro 846 hot swap chassis, that has never happened once.  I do question whether I need the 24 bay capacity anymore now that 8tb drives are so available.  They were acquired when I was using 3tb drives and creating onsite media at a prodigious rate.  I only have dual parity on one of my 24 bay servers.  And that is because I sometimes am away from that location for months on end.  Otherwise I have never needed it.  If I still had duct taped servers held together with a snarl of my own bad wiring, I would have much more use for dual parity. 

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Posted (edited)

One of my servers is built using an HP Microserver Gen 8. It has two parity disks and two data disks in the built-in bays, and four more data disks connected via eSATA. Additionally, I have a two SSD cache pool and a Blu-ray drive inside the main case. The external disks are housed in an ICYCube. The Blu-ray drive is connected to the built-in optical drive port. I chose an expansion card that supports eSATA at 6 Gb/s because I knew that link would be a bottleneck. There are several variations on this card and I chose the one with two internal SATA ports (for the SSDs) and two eSATA ports (I'm just using one for the ICYCube) and the x2 PCIe interface. It uses a Marvell 88SE9235 controller but I've had absolutely no problems with compatibility with unRAID. I attribute that to the fact that it uses the standard AHCI driver:

07:00.0 SATA controller [0106]: Marvell Technology Group Ltd. 88SE9235 PCIe 2.0 x2 4-port SATA 6 Gb/s Controller [1b4b:9235] (rev 11)
	Subsystem: Marvell Technology Group Ltd. 88SE9235 PCIe 2.0 x2 4-port SATA 6 Gb/s Controller [1b4b:9235]
	Kernel driver in use: ahci
	Kernel modules: ahci

I mentioned a potential bottleneck and that is due to the fact that the ICYCube uses a single eSATA connection and a port multiplier but the truth is that in normal use only one of the disks is being accessed at any particular time. The worst case scenario is during a parity check or rebuild,  when all four disks are being accessed simultaneously. However, even that is not as bad as might be feared. I'm more than happy with the speeds I'm getting with all 6 TB disks:

 

5ad396eda3718_NortholtParityCheckHistory.png.3d8b7f1b6d95687edac49d87d36b0fe0.png

 

The ICYCube works well, has an internal power supply and a quiet fan and SMART data and temperature readings are passed through the port multiplier without problems. The only slight downside is that it cost me more for the ICYCube than for the Microserver but I don't blame ICYDock for that - their price is fair, it's just that the Microserver was ridiculously cheap. One thing I would not do is move parity disks outside the main case. The eSATA cable is annoyingly non-flexible and a little too long and the connectors aren't locking but by keeping the server away from children and pets it has actually turned out to be more mechanically reliable than either of my conventional (as in, all in one case) servers where I've been fighting against unreliable SATA cables - I no longer use the traditional stiff, flat, red ones. I've replaced them all with these. They are much more flexible, they have a figure of eight profile, the latches work reliably - and they're blue!

 

Edited by John_M
typos

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The majority off issues where people get data loss from 4+1 systems is because they have no supervision - so when they finally notice that a disk has failed, then they later find that it's more than one disk that has problems.

 

Priority 1 is always to make sure the system has working supervision - and that you get mails with the test results.

 

What is important is that the only way the drives will be able to find bad sectors is if all the disk surface is regularly read. Either by doing a long SMART test, or by having the system perform a parity check.

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Thanks all. As usual, lots to think about. I am in the process of setting up notifications, if and when google allow me a couple of extra mail accounts. Regarding parity ... if i wanted to up my parity drive from 8TB to 10TB is it simply a case of swapping out the drive and letting parity rebuild? If something goes wrong is the 'old' parity drive my potential saviour? I've got these 2 larger drives and want them in the array somehow but dont really want to risk losing anything, thats why dual parity to me is needed. Peace of mind ...

 

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2 hours ago, superloopy1 said:

Thanks all. As usual, lots to think about. I am in the process of setting up notifications, if and when google allow me a couple of extra mail accounts. Regarding parity ... if i wanted to up my parity drive from 8TB to 10TB is it simply a case of swapping out the drive and letting parity rebuild? If something goes wrong is the 'old' parity drive my potential saviour? I've got these 2 larger drives and want them in the array somehow but dont really want to risk losing anything, thats why dual parity to me is needed. Peace of mind ...

 

Sent from my LG-D855 using Tapatalk

 

Sounds like you may not have enough slots for your array. I think biggest mistake people make is plan for too few disks.

 

I'd suggest setting up a new server, this one with enough slots to grow. And add at least a couple disks. Get your data moved over server-to-server. And once the new server is working well you can physically move over the larger disks into the new array, and keep the old server for backups and emergency use. New server can have a low powered CPU and minimal memory, that you'd plan to exchange with the existing server once all is set up. A motherboard transplant is not so hard.

 

If you do look at eSata, be caseful. I've seen some of the eSata units come with longer cables and not work reliably. Shorter cases are not as convenient but they work better. I can't recommend USB for an array disk.

 

In a jam I am not against plugging in a bare drive with sata and power and sitting it outside the case on the floor, turned over like a turtle with electronics side up. Not a long term solution, but if your server is out of the way and will not be disturbed while you complete whatever you are doing, I don't think that is an awful option for a short term need. I've precleared disks that way before, but now always have at least one free slot for preclear or emergency use.

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