3ware RAID Performance
I recently came across this discussion about a poorly performing 3ware array. Aside from being rather horrified of the ignorance the original poster displays of the fancy card he’s already purchased, there’s a few points I wish I could drive into his brain with a rapid-fire nailgun:
1) Software RAID leaves the drive’s write cache enabled. The 3ware cards disable the drive’s cache because they have their own cache, typically with a battery backup. The horrible performance he was experiencing is because he had all caches disabled. Duh!
2) You can mix different size/brand drives on a 3ware array. A replacement drive just needs to be larger than the smallest drive in the array. The controller is smart enough to figure this out and use part of the larger drive.
3) Always get the battery backup unit. Sadly, these aren’t included, and if they were, captain dumbass running an array with zero caches wouldn’t have happened. These cards aren’t cheap, so spend the $150 on the BBU module and make it worth the purchase.
4) It’s better to use the 3ware cache because a hard drive’s onboard cache will wipe if you lose power or reboot unexpectedly. This is why the 3ware cards with a cache disable the drive’s cache and have a BBU.
5) Software RAID is susceptible to cache-related corruption issues because there’s no way to back up a drive’s cache. FUA helps, but effectively renders the cache moot if every transaction issues FUA. (3ware cards can fake FUA back to the OS because the card handles the cache on its own – hence why you should always use the BBU.)
6) Enable queuing for SATA drives. (Unless the IT person that you replaced bought 8 shitty Maxtor DiamondMax 10 drives that tend to lock up when NCQ was enabled under the kind of load the 3ware is capable of.)
Speaking of drive caches, if you look at a simple card like the 7006-2 that doesn’t have a fancy battery-backed cache like the 9550SX or 9650SE do, you’ll notice it (the 7006-2) has an option to enable or disable the unit write cache. This is different from the write cache option under unit policies on the higher end cards. Disabling this would equal the same poor performance.
So why am I writing this here? Hopefully someone will come across this as well (I had searched for “3ware storsave profile”) and not make the same incorrect conclusions as the aforementioned link did. I use several 3ware cards, some top of the line 9550/9650 SATA cards, some 7000-series PATA cards, and they all offer excellent performance.
When configured properly and intelligently, a 3ware card can deliver excellent performance.