Poor Disk performance

[Ayman Altounji]

I have a performance issue with an ML370R (800Mhz, 512Mb Ram, SA 431 Controller, 3 x 18Gb 10K drives, Raid 5).

The disks seem really slow on writes!

I have Oracle 8i installed and we do a lot of imports from other databases (typical dump file size is 1.3Gb). It takes for ever to import.... Strange thing is I can load the same file on my laptop (P700,256Mb ram, W2K Pro) in easily half the time.....

Also, we have a 1650R (512Mb Ram, 2 x 500 Cpus, SA431 with 2 18Gb drives mirrored and one 9Gb JBOD) which has the same issue.

I know the SA431 only has read cache but why so slow?

Any ideas ??

[Ayman Altounji]

Because it's a read only cache... and you are using RAID5 which has write overhead... You should be using a 3200 or other such read/write cache controller for an Oracle server.

[Ayman Altounji]

Thanks on,

I have been doing some more tests on physical write/bytes per cc on the ML370 and get an average of just over 3Mb/cc copying a 1.3Gb raw data file. Or 7.2 Minutes to copy. Supposing that we should be running at 160Mb/cc (The 431 is U3 as are the drives) then in theory we should expect 160Mb/cc on burst, say for argument's sake that averages out to 50% which gives us 80Mb/cc. As we are running Raid 5 and don't have any write cache, shall we deduct another 50%? This then gives us 40Mb/cc. So at 40Mb/cc that should copy our file in 35 seconds.
It currently takes 10 times as long. I understand your argument regarding the write cache, but can it have this much of a detrimental effect?

Incidentally, with the same file on the 1600R (mirrored), I get approx 5Mb/cc. (again way below Ultra2 scsi performance) - the same file on my laptop (PIII 700, 20Gb IDE) I get 3Mb/sec !

Can you point me to any benchmark tests and performance figures re disk i/o & DTR's etc as I can't find any on the Compaq site? This would be most helpful.

Many thanks

Jim

[Ayman Altounji]

Writing to RAID 5 is slow by design because of the parity calculation. This means that each write must be preceded by a read action to determine which parity value to calculate. If there is no write cache on the controller, this can be extremely time consuming.

We moved from RAID5 to RAID1+0 (striping and mirroring) on our MS SQL server system with very good results. We hardly see the drive lights flickering anymore.

OK, RAID1+0 (minimum 4 disks and 50% space waste) or a RAID controller with write cache costs more money, but if you are handling GB+ files on a daily base, I would surely do this extra investment.

We gained another 30% speed increase on diskwrites by adding one extra disk for the database log files. These are written sequentialy. If you don't want to invest in any hardware, try using two of your 18Gb disks as a RAID0 (extra speed) or RAID1 (extra security) set for the data, and use the third disk for the log files.

Lammert

[Ayman Altounji]

lammert
is 100% right !!!
Never use oracel, sql, exchange, Lotus Notes, usw. on raid5!!

[Ayman Altounji]

I disagree. I have multiple servers of various configs and run Oracle 8i and SQL 6.5. RAID5 on the 3200, 4250 or 5300 controllers are fine, but the 431 really crawls. Fairly disappointed with it. My desktop and laptop IDE drives are faster at Oracle writes than the 431 even in RAID 0!!

[Ayman Altounji]

Lammert is quite right. I would also check you have the right cable for a true ultra3 environment (160). I made this mistake if you need the part number I can find it. My setup for SQL 7.0 environment is 3 x 9.1GB 10K (RAID 5)and a single 9.1 GB 10K (RAID 0). The 431 is reasonable but you get what you pay for. Get your RAID config right and I reckon you see an improvement. Good Luck!!

[Ayman Altounji]

I am only getting 1 MB/10 seconds into the business end of a 431. Crawling.
Raid5 with #3 36GB 10K RPM on a dual 1GHtz with 1GB RAM.
Wondering if tossing in another $1,200 for another drive would help.
Probably best to to wimper and get a better drive controller like the 3200.

[Ayman Altounji]

Thanks for the advice guys - went for the following upgrades which gave significant I/O perf lift:
SA5300 with the U3 cable (twisted pairs every 5cm or so..);