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pstavros

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Can anyone tell me from experience what their stripe size is and how the server performs under RAID5? I have a new server here in the office and trying to achieve the best performance for my RAID set.

Thanks in advance for your input!
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My IT Indy

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Do RAID 10 for best performance and redundancy. You lose 50% of your capacity however storage is cheap.

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winkelman

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Indeed. Avoid RAID-5: write-performance is not very good. Disks are cheap, buy one extra and go for RAID-10. Or, buy bigger ones and go for RAID-1. Both options are better then RAID-5 for KMS IMHO.
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pstavros

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OK RAID 1+0 is fine but at what stripe size? 16k - 1024K I know that stripe size plays a huge part in performance and I want optimal performance. If stripe size is to small you wind up with more fragmented data, if stripe size is to big and you have lots of small data entry you waste space. What is the best real world experience?
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winkelman

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Aren't you confusing stripes with clusters? To large clusters and you may end up 'losing' lots of hard disk space if you have a lot of small files (such as with KMS's mail store!).

Stripes have nothing do to with the file system, so whichever stripe size you choose, you will not 'loose' any disk space.

Stripes may affact disk performance, yes. I wouldn't know the optimum for KMS. I think it depends on so many factors (incl. the average message size of your company), that you'd need to test it out yourself. I always pick the default, since it is set to balance out pro's and contra's of too large or too smal stripe sizes.
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pstavros

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I don't think so but I could be wrong.

To understand how a RAID card reacts to different stripe sizes, let's use the most drastic cases as examples. We will assume that there are 2 drives setup in a RAID 0 stripe array that has one of two stripe sizes: a 2KB stripe and a 1024KB stripe. To demonstrate how the stripe sizes influence the reading and writing of data, we will use also use two different data sizes to be written and read: a 4KB file and a 8192KB file

In the array with a 2KB stripe size, the array is happy to receive the 4KB file. When the RAID controller receives this data, it is divided into two 2KB blocks. Next, one of the 2KB blocks is written to the first disk in the array and the second 2KB blocks is written to the second disk in the array. This, in theory, divides the work that a single hard drive would have to do in half, since the hard drives in the array only have to write a single 2KB file each.

When reading back, the outcome is just as pretty. If the original 4KB file is needed, both hard drives in the array move to and read a single 2KB block to reconstruct the 4KB file. Since each hard drive works independently and simultaneously, the speed of reading the 4KB file back should be the same as reading a single 2KB file back.

This pretty picture changes into a nightmare when we try to write the 8192KB file. In this case, to write the file, the RAID controller must break it into no less than 4096 blocks, each 2KB in size. From here, the RAID card must pass pairs of the blocks to the drives in the array, wait for the drive to write the information, and then send the next 2KB blocks. This process is repeated 4096 times and the extra time required to perform the breakups, send the information in pieces, and move the drive actuator to various places on the disk all add up to an extreme bottleneck.

Reading the information back is just as painful. To recreate the 8192KB file, the RAID controller must gather information from 4096 places on each drive. Once again, moving the hard drive head to the appropriate position 4096 times is quite time consuming.

[Updated on: Wed, 16 April 2008 19:28]

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winkelman

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Of course you are correct in stating that stripe size can make a big difference in performance. I was merely stating that you wouldn't loose any hard disk space by choosing an inconvenient stripe size. (As you would by choosing a too big cluster size in the file system. Thus my question if you might be confusing things.)

To my knowledge, RAID controllers will fill up 'left over' parts of stripes. Thus adding another layer of possible performance loss: before writing to an already partially filled stripe, the stripe first has to be read to read the data already there, then the stripe has to be written back including the new data.)

So AFAIK stripe size affects performance, but does not affect 'actual storage space'.

Of course, all AFAIK and I could be wrong in all this and my apologies if I've underestimated your level of knowledge.
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pstavros

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Thank You for you help I really appreciate it and I do understand what you are saying.

Thanks!
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