[Discuss] SW vs HW raid (was raid controller drivers)

[John Abreau]

I don't see how ZFS is a counterexample. I'm aware of ZFS as a software
component of the OS, but I've never even heard of ZFS being baked into the
disk controller below the level of the OS.

An argument about ZFS as an alternative to both hardware and software RAID
would be completely orthogonal to an argument about the relative merits of
hardware vs software RAID. It wouldn't be a counterexample; instead, it
would be an entirely different discussion.



On Fri, Sep 26, 2014 at 6:50 AM, Edward Ned Harvey (blu) <blu at nedharvey.com>
wrote:

> > From: jabr at gapps.blu.org [mailto:jabr at gapps.blu.org] On Behalf Of John
> > Abreau
> >
> > I like to stick with software raid. The argument that convinced me was
> that
> > with hardware raid, the controller can fail, and would have to be
> replaced
> > with an identical controller, which might no longer be available on the
> market
> > by the time your controller fails. Whereas with software raid, the
> controller is
> > just a generic disk controller, and if it fails, it can be replaced by
> any other
> > generic disk controller of the same type (ide, scsi, sata, or whatever).
> >
> > If there are performance requirements for which software raid is too
> slow,
> > then sure, you may need to go with hardware raid. But when software raid
> > will suffice, I feel it's the more reliable choice.
>
> There are pros and cons to both.  John, you seem to think hardware raid is
> faster than software raid, and sometimes that's true, but not always.  For
> example, there are lots of HW raid controllers with on-board write buffers,
> that can greatly improve performance of things like EXTfs.
>
> But my favorite counterexample is ZFS.  The way ZFS performs best is when
> you have a direct attached bunch of plain dumb disks and no smart HBA.  If
> your workload is suitable, you can disable the ZIL for maximum performance,
> but otherwise, use a dedicated log device (SSD typically).  The reason ZFS
> outperforms hardware acceleration is because ZFS has intimate knowledge of
> the filesystem layer.  When something in userland says "Overwrite block
> #7432 of this file," ZFS remaps all those blocks to big contiguous region
> on disk.  Unlike most filesystems, where "Overwrite some block" must
> translate to actually overwriting that block
>
> There are some disadvantages (as discussed) with software raid,
> particularly, making it usable by the bootloader.  And sometimes other
> restrictions, like...  In windows software raid, you can't split the raid
> and expect some random NTFS utility to be able to read it.  (Server fails,
> so you attach the disk to another system to extract files out of it, and
> discover you can't, because the dynamic volume is broken or something, but
> you *could* have booted that disk, if the server hadn't failed.)
>



-- 
John Abreau / Executive Director, Boston Linux & Unix
Email jabr at blu.org / WWW http://www.abreau.net / PGP-Key-ID 0x920063C6
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