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Old 12-13-2010, 03:26 PM   #1
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raid5 with ext4: values for parameters when formatting the raid


recently I started experimenting with raid5. I raided 3 partitions into a raid5 (software) array. Chunk size is 128kB.
After that I formatted /dev/md0 with ext4. I chose the following parameters for that:
blocksize: 4096
stride:    32
stripe-width: 64 (2 * stride)
To test the write speed I issued the following command:
dd if=/dev/zero of=/mnt/raid5/zero-file bs=100M count=10
I get a writing speed of 40 MB/sec. When I issue the command on an unraided target I get write speed up to 65 MB/sec.
After reading this wikipedia article about raid5 I did not expect that the speed might drop under 'normal' rate. I know that there can be penalties if databases are involved or for a lot of random writes, especially if the write size is smaller then the chunk size. But I don't think that the command I issued qualifies for this scenario.
Before I continue experimenting with raid5 I would like some confirmation that I got the basic setup ok.

Q: Did I choose the parameters for formatting right? If my understanding is correct then the following equations should be fulfilled when formatting a raid5:
stride * blocksize = chunk-size
stripe-width = (n-1) * stride # n being the total number of active disks that comprise the array

All partitions of the raid5 reside on separate, identical disks. The unraided partition that I used as reference resides on one of the disks that contains a partition which is part of the array. So it is not a 'different manufacturer' issue.
Old 12-19-2010, 08:21 AM   #2
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So, after some further reading it appears that the options for mke2kfs are ok. Apparently the reduced write-speed is normal with a raid5 array. So it is not my (weird) experimental setup. The reason for this is that before anything is written to disc there is some overhead to compute the parity.


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