Date: Wed, 21 Jan 2009 23:15:26 +1000 From: Danny Carroll <fbsd@dannysplace.net> To: Koen Smits <kgysmits@gmail.com>, freebsd-hardware@freebsd.org, freebsd-fs@freebsd.org Subject: Re: Areca vs. ZFS performance testing. Message-ID: <49771FEE.1070606@dannysplace.net> In-Reply-To: <b072dc420901210112g32b20c8cyedca20c8a2efeff0@mail.gmail.com> References: <20081031033208.GA21220@icarus.home.lan> <F55CD13C-8117-4D34-9C35-618D28F9F2DE@spry.com> <20081117070818.GA22231@icarus.home.lan> <496549D9.7010003@dannysplace.net> <b072dc420901072348n7d094937u89b6d24959f8ae3d@mail.gmail.com> <4966B6B1.8020502@dannysplace.net> <b072dc420901090046v77bc5407xacda5e65fc6ad7bf@mail.gmail.com> <496712A2.4020800@dannysplace.net> <b072dc420901090758o4190be22sa747862d9aa77abf@mail.gmail.com> <4976C370.4030406@dannysplace.net> <b072dc420901210112g32b20c8cyedca20c8a2efeff0@mail.gmail.com>
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Koen Smits wrote: > Areca Support: > Dear Sir, > the only difference is > in JBOD mode, controller configure all drives as passthrough disk. > in RAID mode, you have to configure passthrough disk by yourself in RAID > mode > > in other words, you can use raid with passthrough disks at saem time in > RAID mode but JBOD mode not. > > Me: > So does that mean if I use passthrough, I am not protected by the > cache/battery backup? I ask because there is an option for cache mode > when creating a passthrough disk. i.e. Write-Back or Write-Through > > > So 'passthrough' means that the controller lets the OS see the physical > disks just as they are, but with an invisible cache in between that > buffers operations. This way there is no advantage of the onboard XOR > engine, but you do profit from the intelligent cache, which is the most > important anyway imho. Not exactly. In JBOD mode ALL disks are passed through to the OS. You cannot have RAID. The cache is set to Write-Back. In RAID mode, you can mix raid5, raid6 and Passthrough (which are like JBOD but allow writethrough or writeback cache at your discretion). > JBOD mode is at a disadvantage because in this mode the OS sees one > large drive, and is not able to stripe the data to multiple disks, not > taking advantage of the fact that you have multple spindles available. > Makes sense to me :). No, in JBOD, the OS sees all disks individually. What you are talking about is a concatenated disk set which I don't think has a raid level. > I must admit, I do like these results. Very promising. Me too, although I am not sure if I like the idea of turning off the cache flushes in ZFS. I'd be a lot happier if the Areca card would tell me how 'full' the cache was. I'd also love to know if there was a way for the disk to tell me what the status if it's own cache is. > Further tests would be using an SSD for the ZIL, testing linux and NT, > etc. But let's not go there ;). Nope :-) -D
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