Daisy Chain VS Multiple SCSI Cards?

[badk4life]

(see attachment) I was wondering the performance differences between daisy chaining a HP Surestore 5500 with 4 drives vs running 2 SCSI cards(4 ports) straight to one backup server?

We are currently running 1 HP Surestore 5500 with 4 drives. Each drive has 1 SCSI slot connect to "server1" and the other has a terminator. We have 2 SCSI cards on "server1" with 2 ports each. This way with 4 drives we simply connect one drive to one SCSI port and terminate the rest.

Is is possible to daisy chain? Am I doing this correct? if not what are the pluses and minuses for each setup?

[Joshua Scott]

What Tape drives do you have in the TA5500?

Thanks,
Josh

[David Ruska]

As Joshua suggests, knowing the tape drive type (and the SCSI HBA speed) is needed to know what performance impact daisy-chaining the drives could have.

The benefit of daisy-chaining is in reducing the number of HBAs needed, and the number of host cables needed.

The drawbacks of daisy-chaining can be:
1) Reduced performance. If they share the bus, the share the bus bandwidth. It depends on the burst and sustained transfer rates of the devices to determine how much affect this has. Even if the HBA can source enough data to meet the combined sustained transfer rates, sharing the bus will incur some latacy which could cause the drives to fall out of streaming more, and still impact performance. If you only use one device at a time, then this is moot point (no impact).

2) Posibility of one device effecting the other. If a drive were to hang, it's possible that a reset by the software could also reset another device on the bus.

3) If there's a scsi connectivity issue, you can loose access to multiple drives.

If you already have the HBAs & cables, and do not need to feed up SCSI ports, it would be best to leave it as is.

[Joshua Scott]

Some speeds for you:

HBAs:
A5150A - 80MB/sec
A6828A - 160MB/sec
A7173A - 320MB/sec

Tape Drives:
DDS3/DAT24 - 2MB/sec
DDS4/DAT40 - 6MB/sec
DDS5/DAT72 - 6MB/sec
DLT8000 - 12MB/sec
Ultrium 215 - 15MB/sec
Ultrium 230 - 30MB/sec
Ultrium 460 - 60MB/sec
Ultrium 960 - 160MB/sec
SDLT220 - 22MB/sec
SDLT320 - 32MB/sec
SDLT600 - 72MB/sec

Just make sure that the sum of the throughput can be handled by the HBA. so if you have an SDLT320, 2 Ultrium 460s and an Ulrium 230, you will need at least 182MB/sec of throughput, so you wouldn't want to connect all of those to one port on the Ultra160 card.

Josh

[Alexander M. Ermes]

Hi there.
we use several tape libraries in our hardware environment. At the moment each drive has its own SCSI interface to avoid performance problems. We tested daisychaining, when we configured one of the systems for the first time. It did not really work.
Rgds
Alexander M. Ermes