raid5 performance

[Michael Steele_2]

Hi gorbelny:

First, using 'dd' may not be the way to test performance. This is because 'dd' is going to start reading from address zero and then sequentially read on up. Chances are everything is contiguous and you're going to get better performance from contiguous and sequential reading than random access, defraggmented, non-sequential reads.

Second, you're only reading and not writing.

So its better to 'cp' a large file in order to avoid both.

Third, I never heard of better writes in raid one over raid five. Raid one is used for better reads.

As for the Compaq 431, check your HBA MHz on the controller. You can get 32 and 64 MB transfer rates.

Verify your firmware for the disk array controllers.

General rule of thumb is raid 5 for database and raid 1 for filesystems, i.e., archive logs.

Finally, this is an entry level disk array for entry level servers. So don't expect much if you've been comparing it to other disk arrays.

[Michael Steele_2]

Note: 'defragmented' should read 'fragmented'.

[Hein van den Heuvel]

Michael wrote> Second, you're only reading and not writing.

Actually... the main topic suggest the test is only concerned about writing.
But thinak for reminding me

grobelny,

You are writing to a file, and involving the file system caches. Those will change teh IO buffer size you picked for dd to what the file system write 'update' or 'sync' feels best.
If you can, please repeat the test for raw device access.

Also, yourself, myself and other replies speculate about raid-5 penalty. Let's see whether the basic speed is there by reading (to of=/dev/null).
Again, best done to a raw device as you don't want the data to come from a teh cache.

gregerseni>> With 5 disks:
Read
Read
Read
Read
Modify
Write
Write
This is known as the write penalty.

I believe this to be incorrect, as I outlined in my reply which was poste pretty much at the same time.

For raid-5 to modify a block in a chunk it does NOT have to read all chuncks.
It just needs the target chunk and the partity chunk. Merge in the new data, calculate the new parity and write both.

So the Raid-5 write penalty stays the same, no mattter how many disks there are.

The Raid-5 recovery and rebuild penalty does increase with the number of disks.

Michael>> Third, I never heard of better writes in raid one over raid five. Raid one is used for better reads.

I did hear that. raid-1 only needs to do 2 writes for a simple partial stripe-update.
raid-5 needs to perform 2 read adn 2 writes, and it has to wait for the data read to be able to do the writes. Thus it is (a lot) slower.

For large sequential reads, raid-5 may outperform raid-1 because there is one extra spindle to do track read ahead.
Yes there are more spindles in the raid-1 case, but there are competing over the same job and the result may be a distortion to read aheads.

Michael>> As for the Compaq 431, check your HBA MHz on the controller. You can get 32 and 64 MB transfer rates.

There may be some confusion here.
The PCI bus can be 33 or 66 Mhz times 64-bit wide for a max peak speed of 266 MB/s
See for example:
http://en.wikipedia.org/wiki/PCI-X
And:
http://h18004.www1.hp.com/products/quickspecs/10427_div/10427_div.html

Hope this helps someone,
Regards,
Hein van den Heuvel

[Hein van den Heuvel]

>> I'd surly get better results with software RAID. Am I doing something wrong or it just works this way? Thanks in advance,

Btw... how come you mention SW raid.
Is that what you are using, or just an alternative being considered.
The replies so far have assumed you used HW assist raid as per 431 offerings.

IF you are using SW raid, then your the oberserved speeds are pretty much those to be expected!

Hein.

[grobelny]

No, I'm not using using SW RAID on this hardware. It was just an observation made on other, quite different machines. I guess that on SA431 alone I wouldn't get better results with SW RAID since the bus seems to be limited to 80 MB/s (why? I don't know). But if I distributed the disks across the other two channels of standard Ultra2 controller I would certainly get better speeds. Which is more complicated, but viable option.

[Guido W!tty]

Hola,
i have some problems with write speeds also.
Done a test with a 7 GB file.

copy from server 1 to server 2

with 100MB lan speed: 12 min. 50 sec
with 1000MB lan speed: 13 min. 42 sec

The free RAM in server 2 start at 1 GB and the server says it needs 5 minutes to copy.
After 6 minutes the free RAM drop to 20 MB!!! and the copy time ingrease to 15 minutes.

After the task was done, the free RAM slowly climed up to 900 MB.

hardware:

2 x :
ML 350 G3
2003 server std SP II (clean install)
1 GB RAM
compaq 431 array controller
5 x 18.2 GB UW 3 SCSI in RAID 5 (4 + spare)
GB switch
newest firmware + drivers

Does enyone know how to speed thinks up?
Or do i need RAID 1 ?

In the future it is gonna be a file server, so i need some speed ;-)

Regards,
Guido