Newbie RAID 0 question

[Oliver Fromme]

Bastien Semene <bsemene at cyanide-studio.com> wrote:
 > If I'm understanding well, I can add space with gconcat (8) but there 
 > will be two separate stripings, in your example datas will be stripped 
 > between ad0/ad1 and when space is full between ad2/ad3 ?

Yes.

 > I'm using RAID-1+0 structure, so I think I can do the work with 2 arrays 
 > (old one with datas, new one with datas and more space).

You didn't mention RAID-1+0 in your first posting, only
RAID-0.  That's quite a difference.  If you expect to
receive good advice, you should describe your existing
setup as precisely as possible.

 > Do you see some advantage/disadvantage using the gconcat model instead 
 > of a true striping ?

Both is "true" striping, there's only a difference in
how the data is organized, which can have an impact on
performance.

When four disks are organized in a single RAID-0, this
means that stripes are distributed equally across all
four disks.  In theory this can improve performance,
especially for sequential access.

When two pairs of disks are organized in two RAID-0
volumes which are then concatenated, the stripes for
each of the sub-volumes are distributed only across
two disks.  Therefore there's a lower limit on the
maximum performance that can be achieved (in theory).

In other words:  Adding two new disks using a concat
container will not make the whole thing faster (but
not slower either).  It will be just bigger.

It should also be noted that the new volume will be
less reliable.  Every time you add another disk to an
array without redundancy (no matter whether RAID-0 or
concat), you reduce reliability, because the whole
array will go down when any one of the disks fails.
The more disks, the higher the probability that one
of the disks fails.

Of course, if you have RAID-1+0, then things are a lot
different.

 > Is it planned that geom will add an insert command, or is gconcat the 
 > definitive solution ?

I'm not a geom developer, so I'm not authoritative,
but I don't think that such a feature is planned.
As I wrote previously, adding another disk to an
existing RAID-0 is technically expensive and time-
consuming.  I don't think it's worth the effort.

Best regards
   Oliver

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