[Solved] New Build -- Disk Configuration

[MurrayW]

Over the past month, I have purchased four 2TB drives that I plan to use for my unraid build.

• 2 x 7200 rpm Hitachi Deskstar
  
• 1 x 5400 rpm WD EARS
  
• 1 x 5400 rpm WD EADS
  

 

I am not sure if the WD drives are 5400 or 5900 rpm, but they definitely are not 7200 rpm.  Should I use one of the Hitachi's as the parity and the 2 WD drives + the other Hitachi as the data drives?  Also, is a cache drive recommended any longer?  I've read posts where some have said they don't use one and don't really see the need for one.

 

thanks,

Murray

[Joe L.]

Over the past month, I have purchased four 2TB drives that I plan to use for my unraid build.

• 2 x 7200 rpm Hitachi Deskstar
  
• 1 x 5400 rpm WD EARS
  
• 1 x 5400 rpm WD EADS
  

 

I am not sure if the WD drives are 5400 or 5900 rpm, but they definitely are not 7200 rpm.  Should I use one of the Hitachi's as the parity and the 2 WD drives + the other Hitachi as the data drives?  Also, is a cache drive recommended any longer?  I've read posts where some have said they don't use one and don't really see the need for one.

 

thanks,

Murray

If you will be writing simultaneously to two or more data drives, use the 7200 RPM drive as the parity drive.  If all your writes will typically be to a single disk at a time, it really does not matter.  Your parity checks are limited by the slowest disk involved, no matter what its assigned role.

 

In the same way, writes are limited by the slowest rotational speed disk involved.  Since you have 2 7200 RPM disks, that means writes to one of them assigned as a data disk will be faster if the other is the parity drive.

 

A write to both 5400 rpm drives at the same time will be slightly faster if the 7200 RPM drive is assigned as parity.  In real life, you may not ever notice the difference.

 

[MurrayW]

Over the past month, I have purchased four 2TB drives that I plan to use for my unraid build.

• 2 x 7200 rpm Hitachi Deskstar
  
• 1 x 5400 rpm WD EARS
  
• 1 x 5400 rpm WD EADS
  

 

I am not sure if the WD drives are 5400 or 5900 rpm, but they definitely are not 7200 rpm.  Should I use one of the Hitachi's as the parity and the 2 WD drives + the other Hitachi as the data drives?  Also, is a cache drive recommended any longer?  I've read posts where some have said they don't use one and don't really see the need for one.

 

thanks,

Murray

If you will be writing simultaneously to two or more data drives, use the 7200 RPM drive as the parity drive.  If all your writes will typically be to a single disk at a time, it really does not matter.  Your parity checks are limited by the slowest disk involved, no matter what its assigned role.

 

In the same way, writes are limited by the slowest rotational speed disk involved.  Since you have 2 7200 RPM disks, that means writes to one of them assigned as a data disk will be faster if the other is the parity drive.

 

A write to both 5400 rpm drives at the same time will be slightly faster if the 7200 RPM drive is assigned as parity.  In real life, you may not ever notice the difference.

 

Thanks Joe.  it would be very rare that I would be writing to 2 data drives simultaneously so it sounds like from a speed standpoint, I could use any configuration.  From a reliability standpoint, would I be any better off separating the Hitachi's as a parity and data drive rather than making them both data drives in case there was a bad batch and both drives failed at the same time?

 

Any opinions on whether to bother with a cache drive or not?  Most of the writes will be occasional DVD/Blu-Ray rips, OTA recordings from a silicon dust HDHomerun and scheduled backups for the 3 other computers'laptops in the house.

 

Thanks again,

Murray

[Rajahal]

The updates in the 4.5+ series of unRAID has substantially improved write performance to the array without the need for a cache drive.  Personally, I have seen ~20 mb/s write speeds, whereas others have reported up to 40 mb/s write speeds.  The first is acceptable, the second is exceptional.

 

That said, I still choose to use a cache drive.  Writing to my older cache drive (320 GB Seagate 7200 rpm w/ 8 mb cache), I see speeds of ~60 mb/s on a consistent basis, and they sometimes spike up to ~65 mb/s.

 

Cache drives have a few benefits besides speed as well:

• reducing/eliminating defragmentation of your data disks
  
• can be reassigned as a data drive on the fly to give me a bit of a space buffer - allows me to wait for HDD sales when I run out of space
  

 

While in general I would recommend that you use your smallest drive as a cache drive, there is at least one good reason to use a drive that is the same size or larger than your parity drive.  If you have a sufficiently large cache drive, you can think of it as a 'warm spare'.  This means that if any of your data or parity disks die, all you have to do is stop the array, unassign the dead disk, and reassign your cache drive into the spot.  Then let the rebuild complete.  This gives you near-immediate coverage after a disk failure.