DragonFly BSD
DragonFly kernel List (threaded) for 2004-09
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Re: vinum?

From: Nathan Hawkins <utsl@xxxxxxxx>
Date: Mon, 13 Sep 2004 22:06:42 -0500

Matthew Dillon wrote:

If you want *real* RAID, you want to use an external raid with a SCSI
port. Second choice would be to use a real raid controller. Third
choice would be to use a software solution.

In general, I agree with that. But there are some cases where software can be more flexible. A good example being temporarily breaking mirrors during OS upgrades. I've found that's more difficult to do with hardware raid. (At least the systems I've had access to.) I've also frequently used software raid (Veritas) to temporarily mirror a volume, so I could move it to a different disk. Not necessarily for disk replacement - most often it had to do with keeping heavily accessed database volumes on different disks.

Being able to do things like that online, and having full control over it through the OS is handy.

Doing RAID properly in software requires either having battery backed
ram cache or having reliable off-machine cache (which could be another
machine on its own UPS, frankly). I think the off-machine choice is a
lot more viable these days with the availability of cheap GigE.

The only real difference is where the software runs. At the server, or on a controller. Granted, battery backed cache is pretty much necessary for write performance now that disks are so large. But whether that's even an issue depends on the application.

Vinum is a rather fragile piece of software, it would not be my first
choice despite all the fine work done on it.

The user interface seems to be where the problems are. Most people have problems with the underlying concepts anyway, so the user interface compounds that, and makes it more likely that people will shoot themselves in the foot.

As far as Geom goes... well, I really dislike the idea of having to
implement complex drivers in the kernel. I have been slowly cleaning up
our IO infrastructure to allow IO devices to be properly stacked (our
disk layer, for example, is now properly stacked), but ultimately I think
the real win here will be to form a streaming protocol that could run
over a TCP socket to govern the I/O (not necessarily NAS). Then one
would be able to build drivers to run in userland and/or on remote
machines. The only real latency issue is, as always, with READ ops,
but a kernel supported data block cache at the block device level would
mostly solve that issue.

Geom seems overly complex. I do like the fact that it abstracts out the partition and disklabel stuff, so that it can detect and deal with sun disklabels, Apple partitions, GPT partitions, etc. Having that in userspace seems somewhat less than ideal.

Which leads to the main thing I wanted to point out: Vinum is the only volume manager I've seen on BSD. There's a bit more to it than just raid.


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