How do I view/change physical disk cache policy on PowerEDGE 2950?

Question

Hello,   I need to verify what is the disk cache policy of the physical disks under the control of the Dell PERC 5/i controller in my PowerEdge 2950 servers.   Subsequently, I need to turn off write-back caching on the physical disks (if it is on), and only use the write-back cache of the controller (which is battery-backed), so that there is less risk of loosing data due to a power failure (obviously, the battery backed cache in the controller will not offer any reliability if the write-back cache is enabled in the physical disks, since the physical disk cache is not battery-backed).   So far, I haven't been able to figure out how to use the MegaCLI utility to do this (is there any other utility that I should be using?). For example, the command:   $ ./MegaCli64 -LDPDInfo -a0 | grep 'Disk Cache Policy'Disk Cache Policy: Disk's DefaultDisk Cache Policy: Disk's Default$   does not give any information about the policy actually being used currently (write-back, write-through, etc.).   Also, the command:   ./MegaCli64 -pdInfo -PhysDrv $$8:0$$ -a0   displays some information, but no information about the physical disk's cache policy currently in use.   The disks are Seagate ST3300655SS, and the OS is Linux 2.6.   Just to clarify, I have no problem setting/viewing the _virtual_ disks' caching policy through either the BIOS config utility for the controller or the MegaCLI command line utility. It is the individual _physical_ disks' cache policy that I'm having a problem viewing/setting.    Cheers, - Arash       .

at5147 · Answer

In the RAID BIOS, highlight the VD and then hit F2. That should bring up the properties of the VD. There will be an advanced tab that you should be able to hit space bar on that will allow you to modify the advanced settings, which will included the caches policies.

For more information about the BIOS, please see here:

Dell™ PowerEdge™ Expandable RAID Controller 5/i User's Guide <ADMIN NOTE: Broken link has been removed from this post by Dell>
and look under Menu Navigation Controls, RAID Configuration Functions and BIOS Configuration Utility Menu Options.

ArashA · Answer

Thanks.

I had read the User's Guide, and didn't find anything related to viewing/setting

physical disks cache policy. But I will follow the instructions you provide in your

post.

Please note that under the topic:

Physical Disk Management (PD Mgmt) :: Physical Disk Actions

the User's Guide lists all the actions that can be taken on physical disks, as

follows (but there is nothing about cache policies):

Table 5-4 describes the actions you can perform on physical disks. See Setting Up Virtual Disks for procedures that can be used to perform these actions.

Table 5-4. Physical Disk Options

Option

Description

Rebuild

Regenerates all data to a replacement disk in a redundant virtual disk (RAID level 1, 5, 10, or 50) after a disk failure. A disk rebuild normally occurs without interrupting normal operations on the affected virtual disk.

LED Blinking

Indicates when physical disks are being used to create a virtual disk. You can choose to start or stop the LED blinking.

Force Online

Changes the state of the selected physical disk to online.

Force Offline

Changes the state of the selected physical disk so that it is no longer part of a virtual disk.

Make Global HS

Designates the selected physical disk as a global hot spare. A global hot spare is part of a pool for all virtual disks controlled by the controller.

Remove HS

Removes a dedicated hot spare from its disk group or a global hot spare from the global pool of hot spares.

Cheers,

- Arash

at5147 · Answer

The actions listed below are for physical disks. We want to work with the virtual disk, which will be under the first menu in the BIOS.

ArashA · Answer

I am a little confused now.   Isn't it true that when you set a caching policy on a _virtual_ disk via the BIOS utility or otherwise, that caching policy is associated with the battery-backed cache on the controller?  In other words, it has nothing to do with the caching policy for the cache on the individual _physical_ disks that make up the virtual disk.   I know how to set/view the caching policy of virtual disks. My question is about physical disks, not virtual disks.   Cheers, - Arash

at5147 · Answer

Caching has nothing to do with the physical disks, just the virtual disks. You do not set caching policies for physical disks.

ArashA · Answer

Let me try to explain it more clearly.

We have a server with two components:

1. Dell PERC 5/i controller
---------------------------

- 256M cache, battery backed
- caching policies: read-ahead, write-back, write-through, etc.

"In Write-Back caching, the controller sends a data transfer
completion signal to the host when the controller cache has
received all the data in a transaction."
(from: < ADMIN NOTE: Broken link has been removed from this post by Dell> )

2. Six Seagate ST3300655SS SAS disks
------------------------------------

Each with the following characteristics:

- 16M cache, __NOT__ battery backed
- caching policies: read, write

"If write caching is enabled (WCE=1), then the drive may return
Good status on a write command after the data has been transferred
into the cache, but before the data has been written to the medium."
(from http://www.seagate.com/staticfiles/support/disc/manuals/enterprise/cheetah/15K.5/SAS/100384784c.pdf )

So, there are two caches in play here.

A data loss scenario looks something like this:

the application writes the data to disk;

the controller (in write-back mode) will acknowledge that
the data has been received, so the application thinks that
the write transaction is complete;

at some point in future, the controller decides to flush
its write cache to disk;

the disk acknowledges to the controller that the data has
been written, so the controller thinks all is done;

but the disk is actually caching the data in its cache, while
it decides when to actually write it to the medium (magnetic
disk);

meanwhile, a power outage happens;

the data in the physical disks' cache is lost forever;

If the application was a database, your database will be corrupted.

So, I want to turn off the write cache (16M cache) on the physical
disks.

My question is: how do you do that with the Dell PERC 5/i controller?

Cheers,
- Arash

at5147 · Answer

You cannot do that on the PERC 5/i

AdDeRidder · Answer

Arash - Try this :  'MegaCli64 -LDGetProp -DskCache -Lall -a0 '  to print the logical disk cache policy and this to disable it  'MegaCli64 -LDSetProp -DisDskCache -DskCache -L10 -a0 '  where L10 is Logical disk 10 and a0 is controller 0

Dev Mgr · Answer

The original poster may not even own the server anymore, as his/her question was posted nearly 4 years ago.

rebeloper · Answer

This thread appears on Google when searching for "disk cache policy."

It's important that people are aware that having the disk cache policy set to enable when using hardware RAID may cause data corruption or loss on sudden power failure or shutdown under some conditions.

To disable the disk cache policy on physical disks (not on virtual disks!) you can also do it using the open manage tools.

omconfig storage vdisk controller= vdisk= action=changepolicy diskcachepolicy=disabled

Where is the number the number of the controller and the number of the virtual disk reported by the omreport utility.

<ADMIN NOTE: Broken link has been removed from this post by Dell>

PowerEdge HDD/SCSI/RAID

Was this post helpful?