Hi, thank you very much for a very detailed answer. This machine is in co-location in a specialized data center which provides totally controlled environment (temperature, power etc.) and after being their partner for about 5 years now not a single power outage occured. Of course the data is backed-up regularily.
One more question: where can I see if we are using background fsck? I occassionally run it to check for inconsistencies (as a forground - is this OK?). I suppose I can turn this write-caching on then. Thank you, Matej On Wed, Jun 17, 2009 at 2:48 PM, Erik Trulsson <ertr1...@student.uu.se>wrote: > On Wed, Jun 17, 2009 at 01:35:52PM +0200, Matej ?erc wrote: > > Hi, > > > > we have a HP ProLiant server with RAID 0/1 controller onboard. It is > > detected as mpt0 (I have attached a part of dmesg output at the end of > the > > mail). As reported by some already ( > > > http://www.mail-archive.com/freebsd-performa...@freebsd.org/msg02446.html > ), > > we are also getting extremely slow write speeds. I read somewhere that > there > > are some improvements which could solve the situation in 7.2 (our system > has > > 7.1 installed and I am currently unable to turn it off and it will stay > so > > for at least 3 months). > > > > There are some information that setting hw.mpt.enable_sata_wc=1 solves > the > > write speed (it actually does as I tested!), but I would like to know > more > > about how danger that option is. We are using softupdates and now have > this > > hw.mpt.enable_sata_wc=0, after reading that it might be very dangerous > when > > using sata_wc=1. > > Not very dangerous at all, as long as you are not using background fsck. > The problem with write caching on standard IDE/SATA drives is that they > report that a write operation is finished even if it has only reached the > disk's cache. This means that some of the guarantees that softupdates is > supposed to provide regarding which order data is written to the disk, > cannot be fulfilled. > > This essentially means that if you lose power to the machine unexpectedly > you might have some filesystem inconsistencies afterward that you would not > have had without the disks' cache being enabled. (A normal reset would not > cause this problem since the disks would still retain the contents of their > caches.) > > If you are using background fsck this could be a big problem, since for > background fsck to work properly the only inconsistencies on the filesystem > must be that some blocks are marked as in use when they actually are not. > (That is one of the guarantees that softupdates is supposed to provide, but > may not be able to provide due to the behaviour of the disks' cache.) If > you do have other inconsistencies on the filesystem the whole system may > throw a kernel panic when it encounters one of them. > (A normal foreground fsck would fix all such inconsistencies before the > system starts running for real.) > > It is also the case that if your system is really busy writing to the disks > (with write caching enabled) and you lose power at exactly the wrong time > you could potentially lose a lot of data from the filesystem, since any > given write could theoretically get delayed indefinitely before it hits the > disk's platters. (If the write that gets delayed is the creation of a > directory in which lots of writes happen later you could lose all of them.) > If you have write caching disabled you will not lose more than the last 30 > seconds or so of updates. > > > Using an UPS is one obvious way of drastically reducing the number of times > the machine loses power unexpectedly, and if it is so important that this > server is not taken down I assume you already have an UPS, in which case > enabling the write caching is essentially riskfree. > > > > > > I am really looking forward to getting more information about this, it is > > actually driving me nuts. We have a number of other servers and there are > no > > problems with RAID controllers at all. And as I said, I cannot actually > turn > > of this machine and bring it back to reinstall new OS. > > > > Thank you very much for your comments and thoughts, > > Matej > > > > > > The server model is ML110G5. > > > > mpt0: <LSILogic SAS/SATA Adapter> port 0xd000-0xd0ff mem > > 0xfcefc000-0xfcefffff,0xfcee0000-0xfceeffff irq 16 at device 0.0 on pci5 > > mpt0: [ITHREAD] > > mpt0: MPI Version=18.104.22.168 > > mpt0: Capabilities: ( RAID-0 RAID-1E RAID-1 ) > > mpt0: 1 Active Volume (2 Max) > > mpt0: 3 Hidden Drive Members (10 Max) > > > > -- > <Insert your favourite quote here.> > Erik Trulsson > ertr1...@student.uu.se > _______________________________________________ firstname.lastname@example.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"