Disk Array and LTO Tape support for Mac Pro with limited PCIe slots

[Tim Jones]

Hi Folks,

I was asked in a private email about using a single SAS RAID HBA in a Mac Pro to support both a disk array and a LTO-5 tape drive or library.

Currently, the ATTO R6 family of SAS RAID HBAs is the only viable solution for a single slot. The other raid vendors have focused primarily on disk I/O performance meaning that tape I/O is simply not available, so you can't use Highpoint, Areca, or AOC with tape.

As to the ATTO, while their R-series HBAs support both disk and tape very well, the largest card for a scenario involving an external disk array and tape is the R680. Because that card has 2, 4 channel ports, you're limited to a 4Bay JBOD chassis or a more expensive unit with a SAS expander so that you can connect more than 4 drives to the 4 channel port. In the lower priced solutions (G-Tech, Sans Digital, etc.), the channels are simply passed through to the drives point-to-point. This means that you can only attach one drive per available channel. If you go with a more expensive external drive chassis that uses a SAS expander (Example Unit), you would use only one of the 4-channel ports to feed the disk array leaving the second available for tape.

Therefore, if you can live with 4 drives, then the G-Tech and Sans Digital solutions with the R680 are great (I use a Sans Digital on my system). However, if you require higher drive densities, you'll need to go with a SAS Expander-based chassis to support both disk and tape with the R680.

Tim

[Jonas Rejman]

If one would want to use a point to point jbod enclosure like the Areca 4036?
http://www.areca.com.tw/products/6gsasjbod.htm
and ZFS,

would this work with an LTO-5 simultanously?
http://www.attotech.com/products/pro...=ESAS-H6F0-000

Thanks!

Jonas

[Tim Jones]

If one would want to use a point to point jbod enclosure like the Areca 4036?
http://www.areca.com.tw/products/6gsasjbod.htm
and ZFS,

would this work with an LTO-5 simultanously?
http://www.attotech.com/products/pro...=ESAS-H6F0-000

Thanks!

Jonas

To answer your question, yes - that would work so long as you have two slots - one for the Areca and one for the ATTO. The point of my post was to identify a solution for situations where you only have 1 slot available in your Mac Pro and you need to add a RAID array AND a tape drive / library. In that case, you would not be able to have both the ATTO and the Areca cards installed.

Tim

[Rob Ruffo]

Don't some LTOs work with USB as external boxes? I think LTO is lower than USB 2 anyway, so no card necessary.

[Tim Jones]

Don't some LTOs work with USB as external boxes? I think LTO is lower than USB 2 anyway, so no card necessary.

While were able to get pSCSI LTO-1 drives working with the Ratoc USB - SCSI adapter, most LTO-2 and higher drives fail because the USB to pSCSI adapters are designed for disk I/O - which is generally a burst operation. Tape drives are designed for continuous streams of data and they can cause serious failures in the devices themselves. Basically, the device is designed for a 45% data throughput rate while a modern tape drive will drive them at 90% to 100%. In fact, we've seen interfaces that work fine with disk arrays fail horribly when used to write to VXA, AIT or DLT (let alone high-performance LTO). USB-2 can burst 43MB/sec. This is the equivalent of an LTO-2 drive. LTO-3, 4, and 5 all require much higher levels of throughout, with LTO-5 needing better than 140MB/sec to properly stream non-compressible data.

We really are stuck here until the Thunderbolt to SAS solutions become available .

Tim