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The write cache does not have a battery, so you can lose data during a power outage, which is extremely important on a DB server. If you turn the write cache off, it becomes terribly slow.
Wednesday, March 2, 2011
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That’s how every hard drive works though, right? That’s why if you want both performance and safety, you get a RAID card with BBU cache and disable the on-drive writeback caches.
They’re supposedly coming out with new SSDs with a supercapacitor that stores enough juice to write out the cache on power failure. I’ll be glad when it’s the future.
Comment by Mike Barton — March 2, 2011 @ 11:10 am
Well, I think a lot of people assumed that the SSD didn’t need a write cache to provide its performance since it’s random access. It doesn’t look like the article benchmarked SSDs in a RAID against regular disks in a RAID, so it’s unclear if you get any performance benefit after spending all the extra money on an SSD if you turn off the write cache. He did say that an SSD had about as many writes/second as a RAID 10, but that’s a little hard to compare, since RAID 10 would require four disks, which might eliminate the price advantage of regular disks.
Comment by admin — March 2, 2011 @ 11:32 am