Dell server slow disks

Posted on 2013-06-26
Last Modified: 2016-11-23
I have setup a new dell server, windows runs from mirrored raid and data volume is raid 5. The raid controller has a battery pack and the disks are set to write through. I am using it for WDS and it seems slow creating windows pe images which I believe relates to disk speed.
I understood the raid battery pack negatates the need for write cache on the disks but I may be mistaken Hopefully someone can fill in the gaps for me.
Question by:Sid_F
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Expert Comment

by:Subhashish Laha
ID: 39277628
As you have the battery backup of cache on RAID controller, you should use Write back caching.

Write trough Caching.
When the controller receives a write request from the host, it stores the data in its cache module, writes the data to the disk drives, then notifies the host when the write operation is complete. This process is calles write-trough caching because the data actually passes through-and is stored in- the cache memory on its way to the disk drives.

Write back caching
This caching technique improves the subsystem's response time to write requests by allowing the controller to declare the write operation 'complete' as soon as the data reaches its cache memory. The controller performs the slower operation of writing the data to the disk drives at a later time.
LVL 120
ID: 39277641
if you have the option, set to 75% Write and 25% Read.
LVL 33

Expert Comment

ID: 39278378
Which Dell server?  Which RAID controller?  SAS or SATA disks?  Speed?

If you have an S-series controller, they are (S)oftware RAID and not designed for performance.  If you have an H310, then RAID 5 performance is a "known issue" at Dell for that controller and you would be advised to use a different RAID level.

Also, not all controllers have onboard cache ... only the PERC 5/6, H700, and H710.
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Author Comment

ID: 39279002
The issue was the disk cache policy was disabled. As soon as this was enabled for both virtual disks performance was much better. Its done through the dell open manage console

Author Comment

ID: 39280105
I've requested that this question be closed as follows:

Accepted answer: 0 points for Sid_F's comment #a39279002

for the following reason:

LVL 33

Expert Comment

ID: 39280104
I think you marked the wrong post as the answer.  It should be the following, which suggested enabling the cache for optimal performance (although the 13 year old link provided was useless):
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Expert Comment

ID: 39280106
See previous comment.
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Expert Comment

ID: 39283068
Post http:a39277628 should be the answer ... not that it was a perfect answer, but the OP clearly said he did exactly what that post suggested.

Accepted Solution

Sid_F earned 500 total points
ID: 39283120

The settings I selected under write policy were "write through cache" (they were previosuly at write back) for both disks but this made no difference I noticed the field marked "disk cache policy" was set to disabled so I selected change policy and execute and selected enable disk cache policy and the disk was much faster.
LVL 120
ID: 39283624
just playing devil's avocate here, in post http:#a39277628 it does not state enable disk cache policy!

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