I have written some posts on the issues I have had over the past week with my Dlink DNS-323 NAS. As I wrote yesterday I configured the system to use JBOD to combine the size of the two Samsung 250GB disks and then fill the entire array with data to verify that I would be able to recover the files from one of the disks if the other failed.
The findings are interesting. Please read on.
Once both disks were completely full I powered down the DNS-323 and connected one disk at a time to a computer running Ubuntu 8.04, using a SATA-to-USB adapter.
The leftmost disk showed the following partitions:
|/dev/sdb1||1||66||530113+||82||Linux swap / Solaris|
However, the partition /dev/sdb2 would not be mounted. The error in the log was “VFS: Can’t find an ext2 filesystem on dev sdb2”.
I then switched to the other disk (the one on the right). The partition table looked identical but this time I could actually mount the disk – but whenever I ran ‘ls’ I got a lot of errors saying “cannot access test/D0000220: Input/output error”. The error was caused by files that were on the other disk but were referenced by the file allocation table on this disk. The files that were on this disk were accessible, however.
I then studied what data had been saved on which disk and visualised it. In the image below, each pixel represents two files, each 1MB each in size. The first saved file is the one in the top left and it then goes across and down. Red pixels were saved to the left disk whereas blue pixels were saved on the disk on the right. The original image was 1000 pixels wide, I just shrunk it horizontally to be able to fit within the boundaries of this blog.
The fact that it seems very difficult to recover files on one of the disks means that I will probably stay clear of both JBOD and RAID for my DNS-323. Too bad.