PowerEdge HDD/SCSI/RAID

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difference between write back and write thru?

what is the difference between write back and write thru on the perc/2 controller.

My perc is : cache policy : Direct IO : What is the signification ?
I've an Echange server and i must have the write thru option active instead of having thewrite back ?. Is anyone know why ?

thanks for your answers.
Replies (5)
2 Bronze

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.

Good luck
Dirk Martens
Real Software Belgium
2 Bronze

The reason why write-thru caching is specified is this:

IF !(using(UPS)) = TRUE
AND !(using(battery_on_RAID_controller)) = TRUE
AND power_feed_halt = TRUE
THEN file_corruption = TRUE.

If the program relies on write having completed, as the controller has indicated that the write is completed, and the system loses power with unwritten writes still in the cache, you are going to your backups.

So the question is: how good are your backup/restore/recovery procedures?

how much data can you afford to lose?

if you can't afford to lose data, go with write-through.
If you do want to use write-back (like on my workstation), use a UPS and a battery backup on the cache of the RAID controller.

hth,

Paul
2 Bronze

hey guys--i was researching why on my 3 vmware servers (that i did not install/configure) 2 seem to host vm's fine, while the third has some performance issues when compared to the other 2--mainly when powering down guest vm's or defragmenting their disks through the vmware server console. come to find out, the 2 that work great are set to Write Back. the one that is slow is set to Write Through. am i able to change this setting on the server without losing data? thanks!
2 Bronze

My experience, only use write back, write thru destroyed 4 of my servers, very bad options

that makes no sense that write through destroyed your server.   

Write back is the one that is more dangerous if you loose power and have no battery backup on the cache.     Thing is write through could also loose data. 

The safest is if you turn off ALL write caching and use read caching only.   

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