Date: Tue, 12 Oct 2004 11:11:06 -0600 From: "Kenneth D. Merry" <ken@freebsd.org> To: Roisin Murphy <Roisin.Murphy@gmail.com> Cc: freebsd-hardware@freebsd.org Subject: Re: sata raid & write cache state Message-ID: <20041012171106.GB86646@nargothrond.kdm.org> In-Reply-To: <b21e6cca04101117425d2908eb@mail.gmail.com> References: <b21e6cca041010181932879aeb@mail.gmail.com> <20041011043508.GA72113@nargothrond.kdm.org> <b21e6cca041011090816a1352@mail.gmail.com> <20041011210303.GA78436@nargothrond.kdm.org> <b21e6cca04101117425d2908eb@mail.gmail.com>
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[ your mail client is broken -- you replied separately to me and the list.] On Mon, Oct 11, 2004 at 17:42:55 -0700, Roisin Murphy wrote: > thanks, i hope i'm not running in circles now :) > > > most RAID controllers run in write back caching mode. > > 3. Do you think the raid controller could also keep the info on how > the data is spread across the disk platters? From what i understand, > the write back cache on disks, is there to avoid the disk heads from > having to jump from sector 1 to sector 500 to sector 2, but instead > allow it to go from 1 to 2 to 500. But if the controller would know > how the data is spread, it could use its cache to send the bits in a > preferable order. So the controller itself is like another layer, so > there really isn't any need for cache on the disks. And thats what i > was asking, as you yourself mentioned that you think scsi controllers > usually turn the cache off on scsi disks anyway. So therefor i asked, > what the difference was between scsi and (s)ata disks, when both have > disks write cache off. I do understand the tagged queueing concept > now, but if the cache is off on the scsi disk, and the controller's > cache is used, it shouldn't matter. The main point of write back caching is to give the user faster turnaround for their commands without the application having to support deep queueing. As for the RAID controller, the best it can do is re-order commands by LBA. The RAID controller won't have any knowledge of the physical layout of the disk. Even with write caching turned off, though, a disk can re-order commands with a 'simple' tag attribute. (For SCSI, you can have simple, ordered, head of queue, etc.) So the drive can still re-order the commands that it has queued so it can execute them more efficiently. It just waits to send status to the RAID controller until the command has completed. > > A "dead" array implies that you have two failed disks. > > 2. well, i have to ask that guy i know, who does a lot of incident > response type of work, what exactly happened to all those 'messed up' > (s)ata raid arrays. I didn't know, you could recover from a power > outage situation, where you end up with inconsistent parity. Sure, you can recover from a power outage, you just have to scrub the array when you come back up. You also won't know for sure whether you have some data corruption. > 3. As i said, i wont be running any critical transactional db, that > would end up in an inconsistent state if the power goes out, and the > disks reported all the writes (but instead they were pending in the > disk cache). So the db finished the transaction, but after the > crash&reboot, some tables will be modified and some won't. ALL I'm > concerned about is, that my whole raid setup could die because of an > inconsistent parity. I don't really mind loosing/corrupting few > individual files. No, your whole RAID setup won't be dead due to inconsistent parity. I'd recommend getting a UPS if you're going to do a SATA RAID setup. It'll probably cost as much as one or two of the drives. > > If you want the cheapest solution, and something somewhat reliable, you > > could go with software RAID on SATA disks, and put the machine on a UPS. > > With a UPS, you could run with write caching turned on on the drives, and > > without a battery backed RAID controller cache and not worry about it too > > much. As long as your UPS has enough power to shut down the machine, > > you'll be fine. > > 4. In my experience, my PSU or any flaky hardare in my box breaks more > often then there are power outages in my area :) > > > The only problem with pure software RAID is that you generally can't boot > > off of anything other than a RAID-1. (Even then you may have issues.) So > > your boot disk won't have any protection. > > 5. the boot disks usually don't matter, as long as you backup your > configuration, when its changed... > > 6. anyway, thanks for taking time to explain stuff in detail :) Ken -- Kenneth Merry ken@FreeBSD.ORG
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