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=1.5.16.0
> > 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
>
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