[illumos-Discuss] md aka svm aka lvm

[Garrett D'Amore]

I tend to agree... anyone believing that JBOD concatenation gives a
better sense of reliability probably misunderstands how filesystem
metadata (and potentially even block data for the files themselves) is
scattered around the filesystem, and setting themselves up for failure.

Concatenation as a way to "reduce" points of failure is a mistake.   If
you want reliability, then don't use RAID0 or concatenation, unless
using mirrors underneath or somesuch.

I'll allow that there may be other reasons that concatenation is
preferable to RAID0, but I *suspect* that most people who are doing so
are often mistaken about filesystem optimization.  I suspect that in the
vast majority of cases it is better to let the filesystem lay things out
for you.  (In an ideal world the filesystem would be able to monitor
disk activity and move things around when it finds one spindle more
heavily used than another.)

	- Garrett


On Sun, 2010-08-29 at 15:24 -0400, Kyle McDonald wrote:
> 
> On 8/29/2010 2:53 PM, Haudy Kazemi wrote:
> >
> >
> > RAID0 = striping
> > JBOD = straight concatenation
> >
> > Neither has any redundancy, however the potential impact of a failure 
> > is different.    JBOD failure has the potential of being less severe 
> > than RAID0 failure.  With JBOD, most likely you will only lose the 
> > content of single drive that failed (the remaining content has some 
> > chance of being recoverable).  With RAID0, you lose everything larger 
> > than the stripe width, which means any medium or large files, because 
> > they have been striped across multiple drives.  The smaller files fit 
> > within a stripe, so they should still be recoverable assuming the 
> > drive they ended up on is still working.  (Actually, with RAID0, a 
> > failed drive just about guarantees your medium and large files have 
> > holes in them, while with JBOD those files might have holes in them 
> > because of fragmentation.)
> > Some caveats that apply are the effects of file fragmentation and the 
> > potential loss of filesystem tables/metadata.  In either case, if you 
> > lose the filesystem tables/metadata, you will need to file carve out 
> > anything that remains, and file carving doesn't work very well on 
> > fragmented files.
> >
> The idea that the data on one disk would still be recoverable seems a 
> stretch to me. While it may be readable, in my expeirience with SVM 
> accessing the data is not going to be simple - SVM isn't going to  help 
> you out though dd might. On top of that, while it's not striped in the 
> regular way, there is still no guarantee that all the blocks of the file 
> you're interested in will be on the surviving disk. UFS tries to do that 
> somewhat, but on a long lived FS it's ability to do that will be 
> limited. Even if a file is all on one disk, you have no easy way of know 
> which it's on.
> 
> So the net effect to me isn't that great. I've always stayed away from 
> both RAID0 and Concatenation. While it does decrease the flexibility of 
> space usage, If I've had multiple disks and didn't want to have 
> redundancy and didn't need the the performance boost, I've always just 
> partitioned and made a FS on each disk and mounted them on the system. 
> That's really the only way to salvage one disks worth of data when the 
> other one fails. It's the only way to know what files are on which disk, 
> and ensure that each file is completely on one disk.
> 
>    -Kyle
> 
> 
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