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SA641 - did I need it - is salesman correct?

Silas Murray
Occasional Contributor

SA641 - did I need it - is salesman correct?

My salesman told me I needed a Smart Array 641 controller to improve scsi performance. I have a DL380 & am using raid 5 with 3 disks (havent split the backplane). I am also attaching an external LTO drive.

Having got my hands on the hardware, I realise that I could configure this using the 2 on board scsi controllers, without needing the additional smart array controller, but my salesman insists the additional card is required to improve performance "no-end".

Anybody know if he's telling the truth? Given that the LTO drive will be working out of hours, I doubt the extra spend was justified.

Thanks

Silas Murray
8 REPLIES
Terri Harris
Honored Contributor

Re: SA641 - did I need it - is salesman correct?

Performance will be better with the Smart Array. That said, the primary reason to add a RAID card is to have hardware RAID fault tolerance and recovery.
If you are planning to run the LTO drive off the Smart Array, is it compatible with this controller?
Silas Murray
Occasional Contributor

Re: SA641 - did I need it - is salesman correct?

Thanks for your reply Terri,

The SA641 doesn't have an external connector (the 642 does), and so the LTO drive is now hanging off the DL380's on-board external scsi port. I've disconnected the DL380's array from the internal scsi controller & connected it to the 641 card instead.

Are you saying that I can connect 2 controllers to the array to provide redundancy ... the on-board controller AND the Smart Array 641 card ?? If so I guess there must be a second connector on the internal array that i've missed.

Thanks again

Silas
Senopati
Occasional Visitor

Re: SA641 - did I need it - is salesman correct?

DL380 has a built-in dual-channel SmartArray 5i Plus. With a RAID-5 disk on one channel, connecting the external LTO to the other channel (external) should not degrade the RAID-5 performance, right? Adding SA641 will increase the RAID-5 performance, because it supports Ultra320, while the built-in SA5i+ does not. (assuming your HDs are Ultra320 of course).
ernst limbrunner
Trusted Contributor

Re: SA641 - did I need it - is salesman correct?

hi silas

lots of answers. here is my version:

best configuration would be:

1) split your backplane. have the OS disks in position 0 and 1. have your RAID 5 in positions 2 to 5.
2) in order to split the bus, connect 2 cables to the systemboard and install a terminator (see top panel label).

3) ultra 320 will not give you more performance. having 4 disks on one bus CANNOT overload ultra3 scsi.

4) for shortest backup times do NOT use the external SCSI connector bus use a normal SCSI controller instead (i have several reports from my students about performance problems when a streamer is connected to the smart 5i).

5) installation of the battery pack can improve raid 5 write performance by up to 40% (depends on type of data !!!). expect something between 10 and 20 percent and be happy if its more. the battery pack will enable the write cache.

6) do NOT buy a smart 641.

7) sales people are sales people. technicians are technicians.

best regards from germany
ernesto

ps: i am doing proliant trainings for hp in germany and i also do a special 2-day "SCSI + RAID" training - so i am quite familiar with that stuff.



Leonard Davison
Frequent Advisor

Re: SA641 - did I need it - is salesman correct?

What's being forgotten here is that the Raid controller on-board on the DL380 has NO write cache.
It is 100% Read-Ahead only. Write performance it HORRIBLE with the on-board controller with ANY raid configuration.
The SA641 (with battery module) will be configurable to have a write cache.
So, YES the 641 WILL improve SCSI disk performance over the onboard controller.
ernst limbrunner
Trusted Contributor

Re: SA641 - did I need it - is salesman correct?

hi

the answer from leonard is not correct. if it is a dl380 where you can split the backplane (read original question) then it MUST be a dl380 G3 !!!

the g3 can be upgreded with the BBWC option, giving you write cache ability.

the 641 has more cahe, but as a standard it also ships without a write cache.

the cache size is not really important in this case. if you have write performance problems it is the small number of disk drives and raid 5. you will probably never see more than 150 IOs/second doing writes versus 600 IOs doing reads. (a single disk would give you 200 IOs - unbelievable but true).

you can prove these numbers using the iometer benchmark (www.iometer.org). just make sure the io-depth (screen where you select disk to test) is 40 or higher. the standard setting is one and you will have only one active disk because the IO has to be finished before the benchmark generates a new IO. this is ok for ide, bur not for scsi (multiple IO capability of raid controller plus scsi tagged command queuing).

if you add one more disk and migrate from raid 5 to raid 1+0 you will have 2 to 3 time the write performance you have now.

best regards
ernesto
Leonard Davison
Frequent Advisor

Re: SA641 - did I need it - is salesman correct?

Ahhh....But the on-board G3 controller still does not have any write cache unless the BBWC is ordered and installed.
It will be configured for 100% Read only and is not editable. You will always have lousy write performance no matter what RAID level used (Compared to the use of writeback cache). The performance of the entire raid array will then be only as fast as the disks and subsystem ability to physically write data.
Both the onboard SA532 and add on SA641 are 64Bit controllers, however, the 532 is capable of 160 MB/s of Peak transfer and the 641 is capable of 320 MB/s total; 320 MB/s per channel.
So the question remains, how much more does the the BBWC for the onboard controller cost vs the cost of a 641 with a BBWC and what was stated to the sales rep for a goal of servers usablility? Is Read performance, Write performance, Both or None desired?
If performance is not an issue, then NO you do not need the 641.
ernst limbrunner
Trusted Contributor

Re: SA641 - did I need it - is salesman correct?

hi

please do NOT mix up bandwidth and throughput. of course ultra20 has double the bandwiidth of ultra3 (160 mb/s).

ultra320 disks do not give you more than 50 mb/s. so the max (theoretically) of 3 drives will be 150 mb/s which can be handled by ultra3.

in servers you do not have pure sequential access, so the average data rate of a disk is between 5 and 20 mb/s.

if you have mostly random IOs, the data rate of a disk is as low as 1 to 2 mb/s.
(a disk can handle 200 to 250 IOs (15k drive). let each IO be 8 kb - that gives you 250 times 8 kb - thats 2 MB/s.

on the other hand if you DO have predominantly sequential access, the write cache will fill up in no time and cache administartion will DECREASE performace. once the cache is full, it cannot accept more than you can write to the disks.
----------------------------------------
read cache : sequential reads: good improvement

read cache : random IO: negative influence

write cache : sequential: doesnt help

write cache : random IO : good improvement
------------------------------------------

the benfit of a write cache IS dependant on the raid level. it improves performance in raid 0, 1 and 1+0 environments by 10-20%. in raid 5 you can see 30-40% and in adg you may even see 50%.
------------------------------------------

i din many, many benchmarks with my students (iometer) and the nice thing about the smart controller is that you can change cache settings online - just during the benchmark - shows very nice!!!

-----------------------------------------

additional comment: once you install the bbwc in the dl380g3 you CAN set the read/write cache ratio.

best regards
ernesto