Date: Fri, 18 Oct 2002 22:12:17 +0200 (MET DST) From: j@uriah.heep.sax.de (Joerg Wunsch) To: freebsd-scsi@freebsd.org Subject: Re: Multiple FreeBSD SCSI Hosts Message-ID: <200210182012.g9IKCHoI059995@uriah.heep.sax.de> References: <4.3.2.7.2.20021017212517.00aede00@pop.atomz.com>
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Mike Thompson <mike@atomz.com> wrote: > I'm interested in setting up two FreeBSD systems in a very simple > clustering type configuration around centralized external SCSI storage > devices for hardware redundancy. The specific configuration I'm thinking > about is shown in the simple diagram below: > > +-----------+ +----------+ +-----------+ > | FreeBSD | | | | FreeBSD | > | Host 1 | | External | | Host 2 | > | (primary) | | SCSI | |(secondary)| > | | |RAID/DRIVE| | | > | SCSI r/w | | | | SCSI r/o | > +-----+-----+ +---+--+---+ +-----+-----+ > | | | | > +---------------+ +---------------+ Btw., we run it at a customer's site with two Sun servers, and experience shows this is fairly fragile. It frequently causes SCSI bus problems, like bus resets and lockups. We did it for that customer because they didn't want to spend the money on a full-featured cluster solution. We wouldn't do it again, at least not with parallel SCSI. (SCSI over fibre channel presumably doesn't cause the class of problems we're seeing there.) > 1. Do the FreeBSD SCSI drivers support such a configuration by > implementing the SCSI RESERVE/RELEASE commands to lock access to the > SCSI bus? If so, which drivers for which specific SCSI adapters? If you grep for RESERVE through /sys/cam/scsi/*, you'll see that apparently only sa(4) uses it. Btw., tape drivers (and CD recorders) are basically forbidden in such a situation, since the possibility of a bus reset induced by the other host is fatal for them. (It'll cause the tape to rewind, even in the middle of e. g. a write operation.) > 3. Will the secondary server still experience a corrupted file > system because of write caching by the primary server? Yes. > If so, would it be possible to configure the primary server to write > through the cache? Only by mounting with -o sync, which has a huge performance penalty. > 6. Are there alternatives to this type of scenario that may work > better for FreeBSD? The most interesting idea in this context is `Global Filesystem', a distributed filesystem that deals with cache coherency and proper resource locking. This project started out some day in the past on SGI Irix, then they went to Linux, then they went away from open-source and got commercial. :( I remember rumours that someone here was going to consider a FreeBSD port (a commercial one, i assume). -- cheers, J"org .-.-. --... ...-- -.. . DL8DTL http://www.sax.de/~joerg/ NIC: JW11-RIPE Never trust an operating system you don't have sources for. ;-) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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