Understanding Parity and Cache

[stingray060]

Hello!

 

So, my setup is primarily for home file use, films, etc.

I cannot imagine I would ever put a drive over 2TB into it, so I am led to believe a 2TB Parity Drive would be sufficient?

 

This would be my setup if that is true:

Parity: 2TB HDD

Disk 1: 1TB HDD

Disk 2: 2TB HDD

Would that be a realistic, OK Setup?

 

Now, To my understanding, but please correct me if I am wrong, If one of my 2 Drives failed, to repair it, I would just re-insert another drive of the same capacity? , so the 1TB drive failed, i re-insert another 1TB Drive?...

 

As for cache, id assume you`d use an SSD for starters, but I was wondering if it would be even worth doing as this is only home file use, Currently, 2TB of data on it, 1TB Free.

 

Thanks!

 

 

 

 

[John_M]

Yes, it's a realistic setup. If you already have those disks on hand then fine, use them. But if you have to go and buy them, consider that low capacity disks are generally more expensive in terms of cost per gigabyte than mid capacity disks. Of course, high capacity disks are also more expensive but there's a sweet spot somewhere in the middle (around the 4 to 6 TB mark, at the moment) that you might want to consider. You will need more storage space eventually - everyone does - and having fewer, larger disks is preferable to having many, smaller ones. Higher capacity disks are generally faster and with fewer disks there's less to go wrong.

 

If your 1 TB disk fails you can replace it with another 1 TB disk or a 2 TB disk or anything you might find that's in between those values. The lower bound is the size of the disk that failed and the upper bound is the size of your parity disk. The file system is automatically expanded to fill the disk once the rebuild is complete. If your 2 TB data disk fails you can only replace it with another 2 TB disk. If your 2 TB parity disk fails you can replace it with a disk of capacity 2 TB or greater. So, with your proposed set up, if you wanted to keep a spare it would make sense to make it a 2 TB disk. (Note that there is a way of replacing a failed disk with one that is larger than your existing parity disk, but it involves a "double shuffle" which ends up with the new large disk replacing your parity disk and your old parity disk becoming the replacement for your failed disk.)

 

Most people who have a cache disk use either an SSD or a pool containing two or more SSDs. The original purpose of the cache - to speed up writes to the unRAID server - still applies, though there is now another way to speed up writes called "turbo write" that doesn't need a cache. People who want to use Dockers and VMs find that the cache is useful place to store the files used by those two systems. Whether you have a cache is entirely up to you. You could maybe start off without one and add one later if you decide that you need one.

[pwm]

19 hours ago, stingray060 said:

I cannot imagine I would ever put a drive over 2TB into it

 

Now this sounds like famous last words... 

[stingray060]

20 hours ago, John_M said:

Yes, it's a realistic setup. If you already have those disks on hand then fine, use them. But if you have to go and buy them, consider that low capacity disks are generally more expensive in terms of cost per gigabyte than mid capacity disks. Of course, high capacity disks are also more expensive but there's a sweet spot somewhere in the middle (around the 4 to 6 TB mark, at the moment) that you might want to consider. You will need more storage space eventually - everyone does - and having fewer, larger disks is preferable to having many, smaller ones. Higher capacity disks are generally faster and with fewer disks there's less to go wrong.

 

If your 1 TB disk fails you can replace it with another 1 TB disk or a 2 TB disk or anything you might find that's in between those values. The lower bound is the size of the disk that failed and the upper bound is the size of your parity disk. The file system is automatically expanded to fill the disk once the rebuild is complete. If your 2 TB data disk fails you can only replace it with another 2 TB disk. If your 2 TB parity disk fails you can replace it with a disk of capacity 2 TB or greater. So, with your proposed set up, if you wanted to keep a spare it would make sense to make it a 2 TB disk. (Note that there is a way of replacing a failed disk with one that is larger than your existing parity disk, but it involves a "double shuffle" which ends up with the new large disk replacing your parity disk and your old parity disk becoming the replacement for your failed disk.)

 

Most people who have a cache disk use either an SSD or a pool containing two or more SSDs. The original purpose of the cache - to speed up writes to the unRAID server - still applies, though there is now another way to speed up writes called "turbo write" that doesn't need a cache. People who want to use Dockers and VMs find that the cache is useful place to store the files used by those two systems. Whether you have a cache is entirely up to you. You could maybe start off without one and add one later if you decide that you need one.

This is absolutely Perfect! Thanks alot for the quick reply and such a detailed one,

 

Now, IF i was to get a 5-6TB drive or whatever sized drive, what do you get? I hear of special NAS Drives, Is that better/worth it?

 

Thanks.

[pwm]

30 minutes ago, stingray060 said:

This is absolutely Perfect! Thanks alot for the quick reply and such a detailed one,

 

Now, IF i was to get a 5-6TB drive or whatever sized drive, what do you get? I hear of special NAS Drives, Is that better/worth it?

 

Thanks.

 

It's hard to prove they are better. But the manufacturers explicitly specify that they are intended for 365/24/7 use, and that they have some form of detection to abort and retry writes in case of vibrations from other drives. They are also specified for a decent workload, i.e. number of TB of transfers/year.