Parity Sizing question

[born2ride]

I have been looking at red drives came across an 8tb drive for 300. after reading this- https://lime-technology.com/forum/index.php?topic=54407.0  .  Thinking i should do two 4 tb parity drives , or buy the 8tb drive now then in a few months add the other 8tb drive. Any thought ? or  recommendations?

 

Right now i am running 6.1.6 and a basic key.  going to upgrade to plus

[John_M]

Whether you choose 4 TB parity drives or 8 TB ones depends what capacity drives you anticipate using for your data disks. Remember that you can't have data disks bigger than your parity disks. Other than that it's up to you and what you can justify in terms of cost, but you can certainly start off with one parity disk and add a second one later. You might want to consider the cost per terabyte since it varies a lot across the range of capacities. Large capacity disks (say, 8 or 10 TB) are rather expensive per terabyte, but so are small ones (1 or 2 TB). In the middle there's a sweet spot that you may like to consider. Certainly bear in mind future expansion needs. How many drives can your system (or licence key) accommodate? If that number is small you might prefer to pay extra for the larger capacity ones.

 

You can mount external USB disks using the Unassigned Devices plugin and copy your data from them to the array.

[tdallen]

You could start with dual parity from the outset no matter how small the array.  It's not free, though, so you have to decide how much you want to spend.  Personally 4-6 data disks is probably the point at which I'd go to dual parity.  For other people it's 2-4.  Or 10-12.  There's no right or wrong answer, just an increasing risk of multiple disk simultaneous failures as you add more disks to the array.

 

Just keep in mind that parity isn't a backup strategy.  It's meant to keep your data online and avoid the painful and lengthy process of restoring from backup.  If you don't have a clear backup strategy, I'd spend there first before worrying about dual parity.

[garycase]

Agree with just about all the points already noted.  To summarize ...

 

=>  Clearly dual parity is a major improvement over single parity.  Think of it as extra insurance that very significantly decreases the likelihood of ever losing data during a disk rebuild.  But you CAN start with a single parity drive, if that's more economically suitable to you.

 

=>  The parity drive sets a limit on how large your data drives can be.  On the other hand, it's a waste of money to buy a parity drive (or drives) that is larger than the size of the data drives you plan to buy.  So if you buy 8TB drives for parity; then buy 8TB drives for data.    If you're only going to use 4TB drives for data; buy 4TB drives for parity.  etc.  This applies to FUTURE purchases -- not necessarily the size of drives you have now.  e.g. if you currently have some 4TB drives you can use; but plan to buy 8TB drives in the future; then clearly you should buy 8TB drives for parity.

 

=>  Absolutely agree that RAID in any form -- regardless of the degree of fault tolerance -- is NOT a substitute for a good backup plan.

 

=>  As also noted earlier, the # of drives in your array really doesn't change the benefits of dual parity -- no matter how many drives you have, it will protect any disk rebuild against a 2nd drive failure.    Clearly the more drives you have, the more likely it is that you'll encounter that issue -- but Murphy (of Murphy's Law fame) tends to guarantee that number doesn't really matter 

[born2ride]

If you don't have a clear backup strategy, I'd spend there first before worrying about dual parity.

 

Lol  This is my back up and data all in one.  Being on a budget and finally getting all the component needed to complete one build has been tough.  Now you tell me i need to build 2nd one to back up first one! Oh boy!!LOL  Yeah I know  can never have to many back ups!!  On serious the note - I have most of my stuff scatter on smaller drives, hoping this will allow me to combine all to one disc ,then  back that on to another disc or two in the unraid tower. Most important stuff is home movies and photos,  If i lost my tv shows or movie collection its not the end of world. Just PIA. thoughts?

I most definitely appreciate all the wisdom!

Drives I have now in other machines

plex -3tb,2tb,1tb

home moives and photos 4tb, This one will be moved to new unraid

music 1tb

 

Unraid build -this is what i was planning on

4tb parity

250ssd cache

4tb,,3tb,2tb,1tb

was hoping to add a 8tb for xmas or two more 4tb , Edi:t Now thinking of just going with the 2-4tb.

 

 

Garycase  My thought process was that dual parity would hold more data, less chance of loss.  Thinking how can that one 4tb parity drive can store all the info to rebuild a drive knowing that all the drive totals are large than 4tb.

 

 

 

[garycase]

Dual parity doesn't hold any more data than single parity.  The parity drive (or drives) don't hold ANY data.  They simply provide the technical ability to protect against failure of any of the other drives in the array.

 

If you do not have, 'nor plan to get, a set of backup drives, then I would DEFINITELY go with dual parity.

 

So what you need in your system is TWO 4TB parity drives;  plus as many data drives as you want (up to the limits of (a) your UnRAID license; and (b) the capacity of your system ... not only in physical terms, but also based on the number of SATA ports you have available (you can always add more via expansion cards if needed).

 

For example, using the list of drives you provided ... you could have 2 4TB parity disks; plus data drives of 4TB, 3TB, 2TB, and 1TB.  This would give you 10TB of protected storage => i.e. if any 2 drives failed you wouldn't lose any data; although if a drive DOES fail you should immediately replace it and let UnRAID rebuild your data onto it -- what dual parity does is provide another layer of protection so that if a 2nd drive was to fail during that rebuild, the rebuild would still be successful and you wouldn't lose any data.

 

Notwithstanding the excellent protection a dual parity system provides, you should still absolutely have a backup copy of the data you consider important do not want to take a chance on losing => e.g. your home movies & photos

 

[born2ride]

@garycase. You mentsion "a set of back up drives" I am assume you me external from this unraid system .jusy looking for clarification ,

 

 

 

[garycase]

@garycase. You mentsion "a set of back up drives" I am assume you me external from this unraid system .jusy looking for clarification ,

 

Yes, I definitely mean outside of the UnRAID server.  Simply storing data on a different set of drives in the same server is NOT a good backup -- a major event that wipes out your server would also wipe out your backups.

 

What your individual backup strategy is depends on how YOU want to set it up.  Clearly how much backup space you need depends on how much data you have that you need/want to back up.  I personally backup everything -- I figure if it's important enough to store on a fault tolerant server; it's important enough to backup.  So I have a dedicated backup server that backups up both of my other UnRAID servers as well as some of my personal systems.

 

But if you only have a few TB of "important" data to backup, you may very well be able to do your backups on, for example, a single external USB v3 drive => you can get pretty high capacity external drives these days for very reasonable prices ... 5TB for ~ $125;  8TB for ~ $210.

 

You could also use a set of bare drives that you store in drive cases and access via docking stations.  This strategy lets you use old, lower capacity drives that you've retired from active service in your PC's as backups ... these can even be drives that you don't want to use in an active system because of high reallocated sectors counts; excessive hours; etc. -- but are fine for the very occasional access they'd receive in this use case.

 

The important thing isn't HOW you do your backups;  it's that you DO them 

 

[ashman70]

Another important aspect of backups that is often overlooked is that you should test them from time to time, do a test restore, or validate your backed up data in some way. Its all good to have a backup process, see that its working and get emails that the jobs are completing, but its all for nothing if that backups are no good.

[garycase]

Agree.  There are a variety of ways to validate the backups.

 

What I do is keep checksums for ALL of my data; and backup the checksums along with the files that are being backed up.

 

So I can validate the data on my array with a single right-click, "Verify checksums"

... and I can validate the backup data exactly the same way => just point to the backup folder; right-click; and "Verify checksums"

 

I create and validate my checksums using Corz Checksum running on a Windows machine.

http://corz.org/windows/software/checksum/

[trurl]

...  Thinking how can that one 4tb parity drive can store all the info to rebuild a drive knowing that all the drive totals are large than 4tb.

It can't and doesn't. Obviously.

 

As mentioned, parity contains no data. Parity allows the data for a missing drive to be calculated from the data of all the other drives. If you have more data disks missing than you have parity drives, there is no way to calculate the missing data.

 

See this wiki article. Many things about using unRAID make more sense if you understand how unRAID calculates and uses parity.