Date: Thu, 13 Nov 2008 23:59:27 +1000 From: Danny Carroll <danny@dannysplace.net> To: Jeremy Chadwick <koitsu@FreeBSD.org> Cc: freebsd-fs@freebsd.org, Dieter <freebsd@sopwith.solgatos.com>, freebsd-hardware@freebsd.org Subject: Re: Areca vs. ZFS performance testing. Message-ID: <491C32BF.7020805@dannysplace.net> In-Reply-To: <20081113074301.GA13938@icarus.home.lan> References: <491BBF38.9010908@dannysplace.net> <200811130657.GAA26763@sopwith.solgatos.com> <20081113074301.GA13938@icarus.home.lan>
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Jeremy Chadwick wrote: > On Wed, Nov 12, 2008 at 10:57:58PM +0000, Dieter wrote: > The Areca controller he has can do caching of its own (it has 256MBytes > of cache). Meaning, if you disable write cache on the disks (but not > the Areca controller itself), all of the caching being done is purely > controller-based. The actual disk writes between the controller and the > disk will, of course, be "slow" -- but between the OS and the > controller, things should appear fast. It is entirely possible. I do not know however if the Areca cache works just for Raid or also in JBOD mode. The card can be configured via a web interface (it has it's own nic), via the CLI, or via the BIOS. The only setting I do see is: Disk Write Cache Mode. This is what I have tested. It might have been the Areca cache I turned off, or it might have been the disk caches that I turned off. I hope it is the former, otherwise what is the purpose of having a battery backup unit? If the disks cache the write, then you will probably lose data anyway. I think, once I turn on Raid mode, there will be an option to turn on/off caching in the raid part of the config. The manual shows me that there is an option there, but it only indicates that you can change the cache mode from WriteBack to WriteThrough. But for now, since it's in JBOD mode I cannot access that. > Let me outline the 4 test scenarios (I thought I did this in my original > mail to Danny, but I believe I also said "don't get caught up in > excessive granularity because it'll just confuse people now" -- case in > point): > > - Areca cache disabled, disk write cache enabled > - Areca cache disabled, disk write cache disabled > - Areca cache enabled, disk write cache enabled > - Areca cache enabled, disk write cache disabled [**] > > As I understand it, Danny performed the tests with the [**] > configuration. > The tests should have names: Test 1: Areca cache disabled, disk write cache enabled Test 2: Areca cache disabled, disk write cache disabled Test 3: Areca cache enabled, disk write cache enabled Test 4: Areca cache enabled, disk write cache disabled You did outline these, I thought I was performaing test 2 because I am assuming that when you turn on JBOD mode, you do not get caching on the controller. Once I am sure there is not something glaringly wrong with the FreeBSD side of things I'll run as many of these tests as I can. For now, I think it is only tests 1 and 2. So, my thoughts remain, why was the read performance the same, and the write performance actually marginally better, after I turned off the cache? I did a reboot after I turned off the cache but I did not power cycle the drives. Perhaps that is the answer? Or perhaps simply the Areca controller cannot turn off the cache on the ST31000340AS drives. Or perhaps the cache is ALWAYS enabled and cannot be turned off on the controller. That mean I was doing test 4 as Jeremy suggested. That seems a likely possibility as well. In fact, thinking about it now, it makes the most sense to me. -D
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