SCSI Parity Usefullness

[multiplexed]

When is using scsi parity a good idea, and not a good idea. I assume that parity will constrain throughput, and like in ram, the parity really isn't needed most of the time.

Would parity for scsi hard drives and tape drives make sense, or is it not needed?

 

[Ice Czar]

Bus Parity and Cyclic Redundancy Checking (CRC)

most of the time doent cut it, there is a much longer distance and variable signal quality involved in most cabling schemes than in a memory bus, even ATA has CRC

I cant say what overhead penalty is involved in SCSI parity
but it seems to be a rather simplistic scheme and Id guess that it not much
might try a benchmark or two, but then Id also leave it enabled unless you can demonstrate its not needed through testing

review > http://www.lostcircuits.com/advice/atafaq/
(while it is ATA oriented its still covers signal quality issues)

 

[UICompE02]

I cant say what overhead penalty is involved in SCSI parity
but it seems to be a rather simplistic scheme and Id guess that it not much
might try a benchmark or two, but then Id also leave it enabled unless you can demonstrate its not needed through testing

I can only comment on two specific chips, since that's something that you have to be familiar with the architecture of the chip to know for sure. The LSI Logic U160 (53C1010) and LSI Logic U320 (53C1020/1020A/1030) do not have a performance penalty for calculating, generating, or checking parity on the SCSI bus.

At U320 the parity lines of the SCSI bus are not even used for parity - they're used as a data valid indicator as well as signalling other packetized SCSI protocol related things (except in the message phases), so it's not even an issue for U320 really - all error checking is handled by the CRC value that is part of the SCSI Information Unit packet.