[BBLISA] 10+ TB RAID experiences?

[Dean Anderson]

On Thu, 4 Oct 2007, Scott Ehrlich wrote:

> On Wed, 22 Aug 2007, Bill Rucker wrote:
> 
> > Somewhere around Wed, Aug 22, 2007 at 12:36:21PM -0400, a message
> > from Scott Ehrlich went like this:
> >> Let me clarify -
> >>
> >> I'm wondering what companies, other than Network Appliance, also
> >> incorporate snapshot capability?   Maybe they all do, and use different
> >> wording, or some implementations are better/preferred than others?
> >>
> >> Thanks again.
> >>
> >> Scott
> >
> > EMC with their Clariion/Celerra line provides snapshots also. The benefit they
> > have over Netapp is the snapshots can be saved on a separate volume so the
> > production filesystem is not cluttered with the snapshots. I can attest to the
> > use of the EMC solution as it works very well.
> 
> For my own knowledge, why is having snapshots on their own volume better 
> than cluttered?    Any particular scenarios where a separate volume for 
> the snapshopts works better than cluttered?

Probably should let Bill respond, but I'll try to take a stab at it:

Removing the snapshop to another volume allows most likely allows the
chain of copy-on-write updates on a modified block to be reduced. This
length of this chain reduces the filesystem performance similar to
filesystem fragmentation (I'd assume the chain goes most recent back, so
the 'most recent' change is fastest in that respect).  Keeping those
altered copies of blocks in the middle of the file probably kills disk
performance on sequential reads, in the same way the disk fragmentation
does.

So I suppose at worst they just they keep only 2 copies of the unchanged
blocks, (one on each volume), with a long chain of updates on one volume
and and short chain of updates on the 'fast' volume.  Maybe, if they are
really clever, they can keep only one copy of the unchanged blocks. I
don't know...

The procedure for updating a block of a file is probably like this: 

  The slow volume gets a copy-on-write update as usual. 

  The 'fast' volume, just overwrites the original block of that volume,
which is always "latest".

This makes the fast volume stay as sequential on disk as possible. But
I'm just guessing that this is how it works.


> Also, any other configuration and/or performance differences between the 
> netapp and emc?

I've only used the NetApp.

		--Dean


-- 
Av8 Internet   Prepared to pay a premium for better service?
www.av8.net         faster, more reliable, better service
617 344 9000