D-Link DNS-321 2-Bay Network Attached Storage

Well, it was working fine for the features I was using. Immediately updated to their latest firmware release. Put 2 1TB drives in it, all the backup options (rebuild drive, etc.) seemed fine as I played around with swapping drives out. Then I tried to copy all of my current data over to this NAS box. After about a full day of copying (I have several hundred gigs of files) I went to check the status of the backup.

The backup had completed… HOWEVER, since I had turned on data validation (rereads the destination and source files and compares after the backup) it noted that out of the 1000s of files I had backed up that 12 of them were “not equivalent to the source files”.

I took down the names of the files and then did a hex dump compare of the old and new files. To my surprise the files that were copied onto the NAS box had *exactly* 76 bytes of zero in very specific relative offsets in each file. It was always at hex offsets with the last 3 nibbles of the file offset being in the range of xfb4-xfff that were all zero, in all of the “corrupted” files.

Puzzled, I did some Google searching and found that there was a Linux kernel bug found at the end of 2006 that just happens to exactly match this behavior! The kernel was losing the “dirty bits” (modified memory page indicators) when it was writing to ext2 or ext3 file systems (this box uses ext2). This only happened on certain “chunks” (76 bytes for the Linux case) if they were the 76 bytes that fall at the end of a 4k memory page boundary (the last 76 bytes of a 4k page are… you guessed it!! bytes xfb4-xfff).

The data I was transferring was from a Windows XP machine and this NAS box is internally running.. yep, LINUX! I believe they likely have a version of kernel running on this thing that was silently corrupting my data, as all the issues seem to exactly match my conditions.

That is the WORST kind of data corruption (“silent”) because there were NO error indications at ALL except for when it had done the final recompare, which good thing I had turned that on or I would have NEVER known my data was being corrupted as it was copied to this NAS box!

I notified the D-Link tech support people about this issue, and they responded back saying that they are looking into what is causing the problem (think I gave them a good enough head’s up on this one!)

I promptly returned the box to get my money back and am now running w/ a RAID 1 configuration in my main PC instead of having an external NAS box.

Support notes – I stayed on the phone for the D-Link tech support number for a good 20+ minutes, all I got was the answering service kept repeating “due to a large volume of calls, … ” so I just hung up and emailed them instead. Took them about a week to get back to me (but they did).

Other gripes about the box – the little levers to remove the drives were REALLY hard to use, my thumb got sore after swapping the drives a couple times for doing the failed drive testing.

This review is specifically about the DNS-321 as that was the only one I tested, however the DNS-323 is VERY similar to this box (just basically added a print server), so I can’t say if that one is any better or does the same corruption as this one does (it’s quite possible).

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