[TriLUG] Re: New system configuration thoughts

Kevin Jones mrkevinj at yahoo.com
Thu May 3 21:13:39 EDT 2007


Sorry, I remembered that incorrectly. The problem is in SCSI, not the RAID algorithm. Essentially, every current SCSI implementation is a workaround of this 2TB limitation unless they are using a 16-byte CDB (not a current practice it would seem.) 

From: http://graphics.adaptec.com/pdfs/3759_2TB_WP.pdf

The Problem: The 2TB Limit
Blocks and Bytes
All modern drives organize storage into blocks, which are typically
512 bytes. The Small Computer Systems Interface (SCSI)
standard developed in the early 1980s referenced a structure
called a Command Descriptor Block (CDB) that was 10 bytes
long. The CDB, in turn, contained a Block Number field that
was 4 bytes long. SCSI, Serial Attached SCSI (SAS), and Fibre
Channel (FC) drives are all based on this SCSI standard, as are
most operating system storage stacks. A 4-byte block number
field is large enough to represent 4,294,967,296 unique blocks,
or a total of 2TB of storage if those blocks are 512 bytes each.
When the SCSI standard was developed, the potential of
exceeding 2TB per disk seemed to be a distant problem. Today,
however, this 2TB volume size limitation affects not only internal
drives, but also external disk arrays, such as Fibre Channel
and iSCSI.

Fortunately, the SCSI committee had the foresight to also
define a 16-byte CDB that contained an 8-byte block number,
allowing up to 4 yottabytes (YB) of storage to be referenced.
(This is approximately four trillion TB). Since IBM projects
that the total volume of all online storage on private and public
networks will reach just 1 YB in 2010, a single 4YB logical
drive is as difficult to imagine as a 2TB drive seemed twenty
years ago.
However, most major operating systems and hardware vendors
recognize that it’s important to go beyond the 2TB limit, and
are in the process of adding support for 16-byte CDBs. In the
meanwhile, this paper will explain how to work around the
2TB limit.


----- Original Message ----
From: Lance A. Brown <lance at bearcircle.net>
To: Triangle Linux Users Group discussion list <trilug at trilug.org>
Sent: Thursday, May 3, 2007 5:17:15 PM
Subject: [TriLUG] Re: New system configuration thoughts

Kevin Jones wrote:
> It seems like I read somewhere that RAID 5 begins to really take a
> beating when you get in to higher numbers of drives due to a 2TB
> limitation in the algorithm. All of the high-end arrays I've looked
> at use a different algorithm for providing multiple drive failure
> tolerance and the speed actually improves (supposedly) with higher
> numbers of drives.

Higher numbers of drives means smaller drives means it takes less time
to rebuild a failed drive onto a hot spare, thus shortening the
vulnerability window.

I don't think there is a 2TB limit int he RAID-5 algorithm, per se.
More likely has to do with filesystem sizes and 32 vs. 64 bit operating
systems.

More drives can also have a performance impact depending on your
interconnect technology.

--[Lance]

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