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PERC 5/i RAID Card: Tips and Benchmarks - Page 341

post #3401 of 7150
Is it OK to mix-and-match different drives and perhaps even different speeds for RAID-6?

I've just received my Perc 6/i card and was already to set it up when I thought about that so I'm a little worried.

I think all of my drives are 7200 RPM but I do have a couple of WD "Green" drives... (I also have a mix of Seagate, WD, and Hitachi drives)

I don't care too much about performance (as long as it's decent) but just want the RAID-6 protection.
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post #3402 of 7150
Yeah it'll be fine. But you'll be limited if all the drives are not close in performance.
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post #3403 of 7150
Found this info over at xcpus

1) Partition offset. Your partitions on the array must be offset to a common multiple of both the number of drives minus one, and the stripe width. The easest way to do this would be to delete your partitions off the drive (one-by-one works if you cannot do them all at the same time), and repartition using Vista.

XP, and virtually every O/S and partitioning software of XP's day, by default places the first partition on a disk at sector 63. Being an odd number, and 31.5KB into the drive, it isn't ever going to align with any stripe size. This is an unfortunate industry standard.
Vista on the other hand, aligns the first partition on sector 2048 by default as a by-product of it's revisions to support large-sector sized hard drives. As RAID5 arrays in write mode mimick the performance characteristics of large-sector size hard drives, this comes as a great if not inadvertent benefit. 2048 is evenly divisible by 2 and 4 (allowing for 3 and 5 drive arrays optimally) and virtually every stripe size in common use. If you are however using a 4-drive RAID5, you're SOOL.

2) Stripe size. Your stripe width (stripe size x number of drives - 1; ignoring the parity block) must equal or be evenly divisble by your I/O unit size. XP's default I/O block for file read/write operations is 64KB, Vista commonly uses 1-8MB. In order to accomodate this, you should use a 32KB stripe size for a 3-drive RAID5, or a 16KB stripe size for a 5-drive. Again for 4-drives, you're SOOL.

3) I/O block size. Normally you can't alter this, but if you're using low-level async I/O in your apps, self-written applications, or programs that allow you to tune the block size (e.g. many SQL servers), you should set it to write in block sizes equal to or a multiple of your stripe width. Again, XP's default is 64KB.

4) Cluster size. This is how we force Windows to align each write with the start of a stripe block on the array - you will typically need to format your NTFS partition with a cluster size of 64KB. This divides the filesystem down into discrete 64KB blocks, which in combination with a properly aligned partition, positions each allocation unit exactly in line with each stripe block.

The above recommendations, if all met properly, should allow your RAID5 performance under Windows XP or Vista to skyrocket. However, as mentioned, it only works with 3 and 5-drive arrays, with 4 or 6 drives, they will help a bit but probably not a lot (also you will have to position your partitions manually using a hex editor on a different starting sector, e.g. 3072). You won't get decent performance out of a 4 or 6 drive array unless you somehow manage to configure your application to write blocks aligned with and sized appropriately for the relevant stripe size, which you essentially cannot achieve using Windows itself.

*********

Now, I'll be running a 6 disk array, so it seems I'm SOL on this. I've contemplated doing a raid 50, but I really didn't want to give up 3tb of space.

Does anyone have suggestions on block/cluster size for optimal performance out of a 6 disk array?
post #3404 of 7150
Quote:
Originally Posted by sofakng View Post
Is it OK to mix-and-match different drives and perhaps even different speeds for RAID-6?
You'll likely end up with the same performance as if all your drives were the same as the slowest model, but it should work fine.
post #3405 of 7150
Quote:
Originally Posted by dhahlen View Post
Found this info over at xcpus

<snip>

The above recommendations, if all met properly, should allow your RAID5 performance under Windows XP or Vista to skyrocket. However, as mentioned, it only works with 3 and 5-drive arrays, with 4 or 6 drives, they will help a bit but probably not a lot (also you will have to position your partitions manually using a hex editor on a different starting sector, e.g. 3072). You won't get decent performance out of a 4 or 6 drive array unless you somehow manage to configure your application to write blocks aligned with and sized appropriately for the relevant stripe size, which you essentially cannot achieve using Windows itself.

*********

Now, I'll be running a 6 disk array, so it seems I'm SOL on this. I've contemplated doing a raid 50, but I really didn't want to give up 3tb of space.

Does anyone have suggestions on block/cluster size for optimal performance out of a 6 disk array?
Sounds like a load of BS to me. It's the same tweaks that people use on crappy SSDs to get better performance. The tweaks didn't do anything for good SSDs.

Did the poster happen to post any benchmarks substantiating his wild claims?
post #3406 of 7150
Quote:
Originally Posted by dhahlen View Post
Also, I understand that raid-5 write speeds will be slow due to parity, but I should at least see 50-60MB.
You've got some low standards.

Even if those are 5400RPM drives you should get at least 120+MB/sec sustained writes.
post #3407 of 7150
Quote:
Originally Posted by Stereodude View Post
You've got some low standards.

Even if those are 5400RPM drives you should get at least 120+MB/sec sustained writes.
As I previously mentioned, my speed for writes are about 110-130MB/s at first, and after a few seconds / minutes, it drops to 25MB/s and stays that way until the transfer is complete.
post #3408 of 7150
How many minutes/seconds are you talking about for it to get to the 5% point where the performance drops?
post #3409 of 7150
Quote:
Originally Posted by dhahlen View Post
As I previously mentioned, my speed for writes are about 110-130MB/s at first, and after a few seconds / minutes, it drops to 25MB/s and stays that way until the transfer is complete.
Your RAID5 is set to Write-Through? Write-Through will always give terrible performance.

I have no battery on the PERC 5/i, no UPS, and am using Write-Back on 8x1TB array. I have had power drops during transfers etc.

I just delete the data and restart transfers. (Never CUT very large files for this reason, always COPY just in case) And with bittorrent it has to check the data anyway so you are safe there.

I have never had problems even if the power goes out. Just depends on what you are doing I guess. Mine is strictly a file server.

Also, the WB has to be forced in the BIOS when creating the array. If you create the array in the LSI/Dell SAS Manager, it will default the array to WT even if you check the box 'Force WB'. Hence it has to be done in the BIOS.

I recommend changing to WB otherwise you will always see terrible performance.
post #3410 of 7150
Did a truecrypt flat file drive creation of 100gb. Process took about an hour, would have taken 1/5 the time if the write speed didn't drop to 25MB/s
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