I'd hate not to disagree with someone as grumpy and disagreeable as Ed. Personally, I wouldn't take advice on this matter from someone who "worked with disk staging units for at least a year" and "gave up." (Also, I think Ed is a wet blanket.) I had this thing figured out 4 years ago when we first implemented DSSUs in production. I may not be the biggest NBU shop on the planet, but I back up more than 50TB a week using this method exclusively, so I can tell you that it does work.

 

As far “interleaving”, there is most certainly interleaving at the file system level when you run multiple streams to a DSSU. How Ed can say there is no interleaving and then tell you to watch your disk fragmentation is beyond me. Fragmentation = disk interleaving as far as I am concerned. The point is that the files are non-contiguous.

 

Here’s my proof.

 

 

This is a snippit of a utility called DiskView from SysInternals / Microsoft. The yellow bits are the actual 1K fragments of data on disk for that image file above. The little red dots indicate the beginning and end of file fragments. There are 64 little yellow dots between the red dots indicating my 64K clusters.

 

 

Here’s that same section of disk, different image file. These two streams ran simultaneously last night (along with 6 others) and I can guarantee you that the top image wrote faster, and will destage to tape faster than the image below.

 

Why? Imagine you are bpdupicate.exe requesting the first file back to write to tape. Compared to the 2^nd image, you are going to get a lot more reading done and a lot less seeking as your head(s) cross the disk to pickup fragments. Or, so goes my theory.  There is a utility available from Dell that will show me the amount of time spent reading / writing versus seeking per disk but I didn’t have the time to acquire it and test.

 

Now, I know there are variables here. As I stated before, one of the big improvements to my speed was using a 64K cluster size. Last time I checked this wasn’t available in Unix/Linux. Then again, ext2/3 file systems also like to leave “space” between their writes to account for file growth, which may help (but I doubt it.) I intended to test this several years back, but my management put the kibosh on Linux media servers. The raid controller, simultaneous read/write, spindle count, and disk type also add a lot of variability.

 

I haven't tested any of this on a SAN volume, only on direct attached. I don't think there is much to be gained by taking a 6TB lun and partitioning it at the OS or breaking it into multiple luns at the SAN.  After partitioning, the entire DSSU is still on the same raid group / set, which ultimately controls your performance. If you could take your 6TB lun and break it into 3 x 2TB raid groups / luns then I think that would help. I’ve actually considered breaking my 14 disk RAID5s into 14 single disks for performance testing (single stream each), but that’s an entirely different management nightmare (14 DSSUs per media server etc…) A single SATA disk can drive LTO3, assuming the data is all nicely lined up.  The minute that head has to go seeking, you are in a world of hurt.

 

Again, I would start with a single stream to that 6TB DSSU and see what you get both writing to the DSSU and destaging to tape. Whatever performance you get out of that configuration is your best case scenario. Multiple streams or creating multiple partitions will only drag your numbers down. The crux of the issue (at least for me) is balancing the number of streams I need to run to get my backups to DSSU within my windows, versus the destaging speed I need to get that data off to tape on time.

 

Good luck,

 

-Jonathan