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
_______________________________________________
Discuss mailing list
[email protected]
http://lists.illumos.org/m/listinfo/discuss
