Write-through

[coolrunnings]

Ok, probably a bit of a RAID n00b question but here it goes. I've got a RAID 5 array with 4 320gb Seagate 7200.10 drives on an LSI Logic 8308ELP RAID card. According to what I'm reading, unless I purchase their battery backup unit (LSI00012-F) to the tune of $150 or so, write-through is not able to be enabled. What does write-through do for me and can anyone let me know if it's really worth the $150 price tag?

 

[drizzt81]

I think you may have reversed the terminology:
Write through cache means that any writes are commited to disks immediately. Write-back cache will put writes in the cache (controller RAM) first and then commit them when it is `convenient'. Depending on your access pattern (size and frequency of writes) the write-back cache can give you a significant performance improvement or barely any.

The reason that LSI may require a battery backup unit (BBU) for write-back cache is that it'll make sure that you won't lose data if the power goes out. If your system should shut down while there is still uncommitted writes in the cache, you'd risk file system and/ or data corruption. The BBU will keep the RAM `alive' until you reboot, allowing the controller to commit the write at that time.

Long story short: What is this system used for? If it is a database with lots of small writes, the BBU can buy you quite a bit of performance (given and adequately sized cache). If you are doing video encoding (sequential, large writes) you may as well save the money.

 

[coolrunnings]

Thanks for the clarification!