Adaptec 7805 Raid 6 Performance - 4 Drives?

Posted on 2013-06-10
Last Modified: 2016-12-08

I am trying to understand Raid performance in general.

If I am using Raid 6 it looks like I get:

2x read speed, no write speed gain

However what I am not clear is the baseline for these performance.

Am I correct in assuming that these 2x read gain would be based on the performance of the drive without using RAID

So the read speed is 2x the standard read speed of the drive by itself.

Is that correct - I am assuming that the RAM on the card would also give it an extra boost in performance. Or am I over simplyfing it.

Or would it be the sustained non-cached write speed x 2 for read and write would be the non-cached write speed.

I would be using the SATA - ST4000NM0033 Seagate Constellation ES.3.

So would the read speed be 2x 175 MB/s and the Write speed be 175 MB/s

Max. Sustained Transfer Rate OD (MB/s)


Question by:whorsfall
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garycase earned 333 total points
ID: 39234150
The speeds depend on how many drives in the array.

Since the data is striped across the drives, the more drives involved, the faster the read speed.    Writes are slower, as these require multiple I/O's, since the drives have to first be read; then written (to maintain fault-tolerance).

In general, with RAID-6 the write speeds will be in the neighborhood of 40% of the read speed, but this is also influenced by the onboard cache memory (the "RAM on the card") your controller may have -- if a write operation fits entirely in the cache, the APPARENT write speed will be VERY fast, and the actual writes will take place after the I/O has already been "completed" from the OS's perspective.

Read speeds are very good -- and the more disks, the better.   For example, with an 8 disk RAID-6 your read speeds will be roughly 6 times the speed of an individual drive.
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Assisted Solution

David earned 167 total points
ID: 39234231
"So would the read speed be 2x 175 MB/s and the Write speed be 175 MB/s"
no way will you even come close in the real world.  Every write is going to require a read from 3 of 4 disks.  Since i assume your O/S will be booted here, then you'll be doing a lot of writes 24x7 for logs, journaling, swap, and updating filesystem inodes/descriptors.  

You'll  be lucky to get 1/3 of that as a boot device.  Also blocksize of the raid not equal blocksize of filesystem.

you go R6 to mitigate data loss. if you want speed, go 2xR1 or a R10.  if you want speed & data safety, buy larger drives & go with a 3-way R1. (and software RAiD)... or use SSDs

Author Comment

ID: 39234271

Thanks for the responses - why would it be worse if it is a boot device?

I am planning to use it for VMware ESXi 5.1 as a single volume.


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Expert Comment

ID: 39234279
because of all the random writes the boot device must do in addition to application I/O.
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Accepted Solution

garycase earned 333 total points
ID: 39234336
Agree it's not a good choice as a primary boot drive.   As I noted above, writes are MUCH slower than reads in a RAID-5 or 6 (roughly 50% as fast in Raid-5, 40% in RAID-5), because there are multiple I/O operations involved in each write.

I'd use a good SSD for your boot device (which you can image to your RAID-6 array for backup); and use your RAID-6 for your fault-tolerant data.

... or build two arrays => a RAID-1 for the boot drive; and a RAID-5 for the data.   If you want really FAST you could use SSDs for the RAID-1.

Author Closing Comment

ID: 39237033
Excellent answers thank you.

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