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RAID i/o performance

Posted on 1997-10-20
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Last Modified: 2013-12-16
Hiya...
Running Solaris 2.4 on Sparc-20.
We have a situation where our RAID array on sd8 which is Fast and Wide SCSI, is very very slow,
here is output from iostat.

#iostat -xtc
                                 extended disk statistics       tty         cpu
disk      r/s  w/s   Kr/s   Kw/s wait actv  svc_t  %w  %b  tin tout us sy wt id
sd15      0.2  5.2    0.2  237.4  0.0  0.1   19.9   0   9    0   96 19 25 52  4
sd16      0.0  0.0    0.0    0.0  0.0  0.0    0.0   0   0
sd17      0.2  6.2    0.6  346.9  0.0  0.2   33.6   0  21
sd3       1.8  0.0    8.0    0.0  0.0  0.0   14.0   0   3
sd8      17.2  9.4  577.7  292.4  1.8  2.3  156.2  62  85


##This is an entry in our /etc/system file
set ufs_ninode=5000
set ncsize=10000
set sd:sd_max_throttle=10
set scsi_options=0x178


Now heres the question.
Why does the Kr/s added to Kw/s not add up to the expected 20-40 megs per second. also what are the "set" options doing? and what impact do they have on the performance
of the Raid.
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Question by:rickyr
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by:gabsi
ID: 1812721

The RAID is a secured solution, not a performant one.

Using RAID, the system has to compute parity bits each time it access disk.
It has also to distribute data on several disks when it writes, and
to read on several disks.
If your raid system has no memory cache, it can't be performant.
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Author Comment

by:rickyr
ID: 1812722
Hiya...

The raid has 32mb memory cache. It is a 108gig unit. The speed of the i/o is very very low, and our RAID suppliers are looking into this for us.

Meanwhile could you answer the second part of the question, about the /etc/system file, Are the entries relevant to the RAID? What do they mean?
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Expert Comment

by:wcarson
ID: 1812723
Your problem may be much deeper than this, but to achieve greater than 20 M bits a second transfer rates your SCSI cabbling must be
as short as possible. I have seen performance loss in cables as short as 1 1/2ft. With an array as large as yours, your cable may be pretty long.

Unfortunately, RAID problems can be very difficult to trace.
Chuck
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by:rickyr
ID: 1812724
Hiya...
OK so the RAID problem may be a too deep to get into here. Our suppliers are looking into this for us anyway, so we'll forget about that. I'll bring up cable quality with them though, (Thanks for that).

Any ideas about the "sd" statements in the "etc/system" file. Do these have an impact on performance?

regards
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Author Comment

by:rickyr
ID: 1812725
Hi...
We stripped out the set scsi options, and thru-put is now around 4 megs a second (sum of reads and writes). As we are getting closer to the solution, can anyone now state the correct scsi options to attain the full potential of the i/o (8-10 megs a second, I am told). This is a mission critical setup and we have a couple of running theories, so could anyone help here PDQ, before this solution is found by ourselve or our suppliers.
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Author Comment

by:rickyr
ID: 1812726
Hiya...
Running theory no. 1& 2 were...
set scsi_option=0x100 and...
set scsi_option=0x200 but...
neither worked, they just took the i/o perf back to how it was in the first place.
regards

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Author Comment

by:rickyr
ID: 1812727
err!...
I get the feelin, "ain't nobody here but us chickens"!
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Expert Comment

by:john_jenkins
ID: 1812728
Yo
I've seen a similar problem where one (or both!) of the SCSI terminating resistor packs were wrongly fitted or missing - it's worth a check, tho' this does tend to give intermittent other SCSI faults (typically tape drives 'going away' until a reboot) as well. Also make sure there are no hardware configuration contention problems with your SCSI controller and another device - e.g. do you have a 2nd SCSI controller ?
Regards
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by:rickyr
ID: 1812729
Hi...

Our RAID suppliers have sorted this out for us. We had to serial link into the RAID from a PC then enabled write cache and a the blocking size.

Thanks everyone
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Accepted Solution

by:
linda101698 earned 100 total points
ID: 1812730
I'm posting the solution so it can be saved in the PAQ.

                          Our RAID suppliers have sorted this out for us. We had to serial link into the
                           RAID from a PC then enabled write cache and a the blocking size.
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Author Comment

by:rickyr
ID: 1812731
Hi

Just to add.......

WARNING!!!!! Enabling write thru cache will corupt your data if there is a power outage, unless the RAID has an internal backup battery for it's cache'ing, or the whole system is on a UPS (Uninterupptable Power Supply). this is so that the data held in cache can be written back to disc.

regards
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Expert Comment

by:linda101698
ID: 1812732
Thanks for adding the warning rickyr.
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