JBOD: 2 Drives in Single Drive Letter Question

[Dave3000]

Suppose I had a JBOD hard drive configuration that combine 2 drives in a single drive letter. If I was transferring a huge file and the first drive in the JBOD ran out of space would the RAID controller write the rest of the file to the 2nd drive? If this is true can this cause issues if it's a movie file spanned across the 2 drives such as a 5 second stutter when it start to read the rest of the file on the second drive. Let's assume sleep mode was enabled in the drives and the 2nd drive was in sleep mode.

 

[dave_the_nerd]

JBOD mode is "just a bunch of disks." Each drive would be a separate mount point, it's own volume. When you run out of space on the one drive, that's it. Copy fails.

What you're describing is called Drive Spanning:

http://mintywhite.com/vista/span-volumes-widows-drives/

RAID controllers and most NAS's don't support it. It's generally a terrible idea, since it combines the worst aspects of single-drive performance with the crap reliability of RAID-0.

In the situation you're outlining, if you were using drive spanning, both drives should be spun up all the time. If one fails to respond in time because it's asleep, the volume will take a dirt nap.

 

[Dave3000]

Well a few days ago I returned a QNAP TS-288 NAS. I had my drives configured as JBOD on that NAS but it did not show 2 drive letters. It only showed folder names and I did not have a choice of a drive volume. There is a possibility that I might purchased the 251+ and I was wondering if JBOD is setup as separate volumes for each drive installed on that NAS?

 

[dave_the_nerd]

Well a few days ago I returned a QNAP TS-288 NAS. I had my drives configured as JBOD on that NAS but it did not show 2 drive letters. It only showed folder names and I did not have a choice of a drive volume. There is a possibility that I might purchased the 251+ and I was wondering if JBOD is setup as separate volumes for each drive installed on that NAS?

NASes work differently - they share folders, not volumes. Each folder that you accessed was located on either one drive or the other.

 

[Dave3000]

So in a NAS what happens if a large file is too big to fit on the rest of the space of the first drive? Does it write the rest of the file the the second drive? Also in a NAS do both drives keep spinning in JBOD regardless of which drive is being used? Are they spinning/sleeping in sync in a JBOD?

 

[dave_the_nerd]

So in a NAS what happens if a large file is too big to fit on the rest of the space of the first drive?

Nothing. You can't write the file. Each drive is a separate file system.

Does it write the rest of the file the the second drive?

No.

Also in a NAS do both drives keep spinning in JBOD regardless of which drive is being used?

Depends on the power/sleep settings you have.

Are they spinning/sleeping in sync in a JBOD?

Same. Just depends on the power settings and the NAS.

 

[Dave3000]

Nothing. You can't write the file. Each drive is a separate file system.

What are you suppose to do then? Will it automatically write the whole file to the 2nd drive in the NAS transparently?

 

[dave_the_nerd]

What are you suppose to do then? Will it automatically write the whole file to the 2nd drive in the NAS transparently?

No. The other drive has its own folders. Or none. Depending on how you configure it.

There's a layer between you and the drive called the "file system." In pretty much every circumstance, a file can only be contained within a single file system.

Underlying file system is the "volume". This can be a partition on a disk, a whole disk, or some kind of multi-device volume - a RAID array, a spanned volume, whatever.

Let's say, as a for-instance: NAS, 2 6TB drives in JBOD mode. Drive1 and drive2. You format them both, and make a shared folder one drive1 called share1. That share1 folder exists within the 6TB file system on drive1, and that's all she wrote.

If you copy more than 6TB of data, you will run out of space on drive1 and the NAS will say "screw off, buddy." You would have to create another shared folder on drive2 (share2?) and copy data to that instead. It won't dynamically write data to both shared folders or file systems, since as far as the NAS is concerned, they are completely different entities. None of this is automatic. It's "just a bunch of disks" (JBOD) mode. They're pretty stupid and require you to manage them.

Now, if you did somehow create a spanned volume, or a RAID-0 volume (neither of which is JBOD mode) then you would have a single 12TB volume. Writing >6TB of data would be fine, and the data would be stored on both disks as needed, since it would all be in the same file system. But, if one of the two drives fails, your file system would become corrupt and your data on both drives would go away.

The way around this is to create either a mirrored volume (which has half the capacity because it keeps copies of everything on both drives) or a striped volume with parity (which requires three or more drives.)