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I have been saddled with some servers with crap Dell H310 and H330 controllers (which don't have BBWC and provide about 15mb/sec write performance on spinning disks).

I can upgrade the controllers to H710's (which do have a BBWC, but most likely require a rebuild as they apparently require a RAID setup, while I'm using software RAID), switch to SSD's or both.

Is there a substantial IO performance increase when using BBWC on SSD's ? (I note that the system I have running an H310 and SSD's performs hugely faster - I'm not sure if switching to a BBWC based controller on that system will help significantly, and I'm trying to ascertain where to upgrade from spinning disks to SSD and where to upgrade the controllers)

davidgo
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  • Anecdotal and not quite the same thing, but I've heard of big enterprise arrays where solid state without write cache was slower than spindles with. Although all that probably says is that their design was not optimized for super low latency, and that there was a lot of spindles to keep up with the IOPS. – John Mahowald Aug 15 '16 at 01:43
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    Are you asking about battery-backed specifically, or cache in general? I'm not sure about Dell, but IIRC on HP RAID controllers, you needed cache (battery-backed or not) in order to use RAID 5; with no cache, you only had the option for 0, 1 or 0+1. – DarkMoon Aug 15 '16 at 01:47
  • @JohnMahowald - I wonder if that was in a RAID5 scenario ? – davidgo Aug 15 '16 at 02:46
  • @DarkMoon - BBWC - the only controller available for these boards with Cache is the H310 - which has a BBWC. It seems to me like the H310 disables all cache even for no RAID ! – davidgo Aug 15 '16 at 02:49
  • @Darkmoon - sorry, I meant 710 above. Can't edit the comment ! – davidgo Aug 15 '16 at 02:55
  • @davidgo you should be able to delete comments with the little cross at the end of your comment when you mouse over your comment. Then put in what you meant to say – BeowulfNode42 Aug 15 '16 at 03:21

2 Answers2

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Battery-backed write cache allows you to acknowledge writes as soon as they're received into cache. This is faster than SSD. It won't help you on reads, though, and any time your cache is full and the client is still writing, you'll go back down to the speed of the disk.

If your workload is in any way read intensive (database, file server, applications), then prioritize SSD over a new card. If it's infrequent writes that could be absorbed by a battery-backed write cache, then prioritize that. The ideal would be both.

Basil
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  • Any idea what kind of difference in performance one can expect on a (non RAID sata) SSD with BBWC over one without cache ? – davidgo Aug 15 '16 at 02:50
  • For copying VMs, you'll have better performance with the cache until it fills up. Then it'll just be the speed that the cache can drain to disk. So basically the benefit of the cache won't be noticeable for most of the transfer. – Basil Aug 15 '16 at 02:58
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You should have controllers capable of BBWC/FBWC even if you're using SSD.

But specifics matter. What disks? What specific controllers? What's the performance requirement?

ewwhite
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  • H310 controller at the moment (no cache), compared with H710 (with BBWC and 512meg cache). Disks are anything from 2TB enterprise SATA disks to SSD's where I use Intel and Samsung Pro. Performance requirement is not really defined - but copying VM's (nominally 30 gigs) while using LVS snapshotting should not bring the server to its knees. I'm trying to get a feel for the kind of improvements going from HDD to SSD and from SSD to SSD+BBWC will make at this point. – davidgo Aug 15 '16 at 02:54
  • You need write cache. The controller upgrade + BBWC will accomplish this for you. – ewwhite Aug 15 '16 at 03:23