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How to change cache policy without loosing data.
Hi everyone,
I have a T310 server with 2 disk on
RAID-1, my problem is that I have the cache write policy to Write-back and my
UPS has stop responding, so in the meantime I like to change the cache write
policy to write-through and leave it in that way.
My question is, ¿It is possible to change
the cache policy from write-back to write-through without losing any data or
without reinstalling the os?
If it’s possible..¿Can I simple change it
from the bios on the advanced option without deleting the arrays of disk? ¿it’s
possible to change this directly from the OS with any Dell software for Raid
controllers?.
Thanks for the answers and sorry for my
english.
Regards.
theflash1932
7 Technologist
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16.3K Posts
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March 21st, 2012 12:00
Changing your cache settings will not affect your arrays or data, nor is a reinstall of the OS required. Cached data is simply data written to the cache memory to be written later to the disks, reporting the write as complete - this improves a systems responsiveness and performance. Turning the cache off (or changing to write-thru) only forces the controller to commit the write to disk before reporting the operation complete and accepting the next write operation.
As long as you have a RAID controller with battery-backed cache (PERC 6/i and H700) and no RAID memory problems that have been reported, that cached/unwritten-to-disk data remains safe, as it is kept alive in the cache memory by the battery power, which usually has a 72-hour charge.
So, it is not necessary to change it while you resolve your UPS issue, but if you want to, you can change it in either the CTRL-R utility during POST/BIOS or from the OS using OpenManage Server Administrator.
DELL-Geoff P
990 Posts
1
March 21st, 2012 13:00
The write cache is on the hard drives themselves. If you look at the hard drive properties and policies, you will find this function. You have 2 options, enable write caching on the device or Turn off Windows write-cache buffer flushing on the device.
Cache flushing
By default, Windows employs cache flushing. This means that the system will periodically instruct the storage device to transfer all data waiting in the cache to the principal storage media. When you select Enable write caching on this device, you turn off these periodic commands to transfer the data. Not all devices support all of these features.
More information on preventing data loss
Systems that have write-caching enabled at any point in path between the application and the storage device must be stable and not prone to power loss. When a device attached to a system uses write-caching, the device’s algorithms for caching are designed with the expectation that power will continuously be available to operate both the cache and the movement of the data in and out of the cache. If your system or power source has known issues with sustaining power, you should not use these features.
theflash1932
7 Technologist
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16.3K Posts
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March 21st, 2012 13:00
No, there is no write cache on the S100 RAID controller:
All SATA disks have a small amount of cache to help improve performance. In most cases, this cannot be turned on/off without a special interface (tool directly from the drive manufacturer - WD, Seagate, etc. - or the BIOS). If you cannot turn it off in the BIOS, then 1) you might not be able to edit the drive's ability to use the write-cache, or 2) it might not even be used at all (on-disk write cache is ignored by PERC 6/i and H700 controllers). If you can't turn off the cache at the disk level (if it is even being used), then it is no worse than a desktop losing power.
clusterworks
5 Posts
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March 21st, 2012 12:00
Hi theflash1932 in what option i can change this setting from the bios? the bios is a PERC S100 but i can not find an option to change the write cache policy.
theflash1932
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16.3K Posts
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March 21st, 2012 12:00
There is no write cache for software RAID controllers (S100/S300) or for the PERC H200 and SAS 6/iR hardware RAID controllers. You only need to worry about this if you have a PERC 6/i or H700.
clusterworks
5 Posts
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March 21st, 2012 13:00
Many thanks for all the answers.
clusterworks
5 Posts
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March 21st, 2012 13:00
theflash1932 ¿so to clarify there is not write-back or write-through on S100 controllers?
¿so when you write the data to the disk this is making directly without any cache?
I have an option on the bios that says Sata cache policy enabled ¿what is this? the bios is not allow me to change it, do you think that with the software OpenManage Server Administrator maybe i can change this parameter to disable?
I don't know when I going to replace the UPS its not depend on me so i don't want to lose any data if the power shutdown without notice.