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set up disk cache

Posted on 2010-11-18
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Last Modified: 2012-05-10
in some articles, they say that you can set up the disk cache for read/write 50/50.
I wonder if this can be set up at the RAID software or at the Bios, or where...?

Thanks
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Question by:jskfan
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rsimsee earned 125 total points
ID: 34165483
Hardware disk caching is controlled by HDD controller, and if you have RAID, than you're talking about the RAID controller.  On most RAID controllers today (if it supports setting the options at all) you can either set the disk caching in the controllers BIOS accessible during the boot process, or you can use whatever RAID management software came with that particular RAID controller.

Hope this helps!
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by:dlethe
dlethe earned 250 total points
ID: 34167440
Hardware disk caching is a standard feature in SCSI, Fibre channel, SAS, SATA, and even IDE drives as well.  In disks that speak SCSI protocol, Mode page #8 has literally a dozen flags relating to it, including the WCE bit (which stands for write cache enable).

SATA/ATA commands require the SET FEATURES command to control the enable write cache bit.

I am not aware of any BIOS that programs these bits in disk drives.   RAID controllers (some) RAID controllers set these in the disk drives as well, but this is also extremely rare.  The cache referred to by RAID controllers is typically the internal buffer RAM within the controller itself, and not the HDD.

To permanently change these settings in disk drives, you need a "Mode page editor".  I'm not aware of a freebie, and you also have to hook the disk up to a JBOD controller, (direct attach to a NON-RAID) controller to send the command, as it would be blocked by a RAID controller.

I would personally, just see if my RAID controller had such a feature and enable it ... IF you have a battery backup on the controller memory board, otherwise you risk data loss.
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Assisted Solution

by:Gerald Connolly
Gerald Connolly earned 125 total points
ID: 34176412
Most modern RAID controllers use intelligent caching algorithims that look at the current workload and allocate the cache accordingly ie if the workload is very read intensive it will allocate on average 75% for read and only 25% for write and vice versa if the workload is write intensive, figures are for illustrive purposes only YMMV.

Are you asking this question for a reason? What problem/issue are yoy trying to solve?
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LVL 47

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by:dlethe
dlethe earned 250 total points
ID: 34176447
Plus there are cache buffers that the O/S reserves for each logical device (which may or may not be a physical device).

So be very specific on what you want to do, or you'll just get confused with too many options. :)
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