Hi Joe
That's an interesting idea, and I (and, I'm sure, others) have spent some
time thinking about it.
Dynamic Drive Sharing with tape devices is based upon several principles:
o only one system can access a tape drive at one time.
o but multiple systems can access a tape library at one time (with each
system accessing different tape drives).
o all of the SAN-connected systems sharing the tape devices can 'see' the
tape drives on their SCSI buses.
o almost certainly, these systems see the tape devices all of the time.
o without some coordination, these system would overwrite each other's data
on tape. NetWorker provides that coordination.
o NetWorker's on-tape format is common, no matter which operating system the
NetWorker software is running on. So, this NetWorker on-tape format is like
a common file system; every system can read and write a tape written to by
an other system. Thus, they can share not only the same tape libraries and
tape drives, but the same tape media, too.
Let's compare that to disks:
o only one system/process can access a disk drive / LUN at one time [unless
a LUN /file system sharing mechanism is used; more on that below].
o but multiple systems can access a disk storage system at one time (with
each system accessing different LUNs).
o all of the SAN-connected systems sharing the disk devices can 'see' them
on their SCSI buses (if we allowed this, which we don't).
o almost certainly, these systems see the disk devices all of the time (if
we allowed this, which we don't).
o without some coordination, these system would overwrite each other's data
on disk. This coordination is provided both by zoning in a fibre channel
switch, and by access control in the disk storage system.
o NetWorker's 'file type' format is common, no matter which operating system
the NetWorker software is running on; it's the same format as the on-tape
format. BUT, these savesets are written as files in a file system, and this
file system must be one of the ones supported by the operating system doing
the writing. On Solaris, this would likely by 'ufs', and on Windows 2000,
this would probably be 'NTFS'. So, it would not be possible to share this
same LUN between Windows 2000 and Solaris, because the file system would not
be usable by both operating systems.
Ignoring this file system problem for a moment, we could allow the same LUN
to be mounted on one system, used for a while, unmounted, then mounted on
another system. Obviously, this is what we expect in a tape world, but it
sounds unattractive in a disk world.
In order to share a disk as we share a tape with DDS, what's needed is a
'common' filesystem on a LUN that can be shared by multiple systems,
preferably by multiple operating systems, with each one mounting, using,
then dismounting the LUN.
Even better would be a file system that could be shared by multiple systems
AT THE SAME TIME. While I don't think this capability is widely and
affordably available today, it is coming. See
http://www.byteandswitch.com/document.asp?doc_id=24108 for a summary of the
several available options. You'll notice that some require a separate
system to coordinate the access to the shared file system. And at least one
makes claims to support multiple hosts reading and writing the same file at
the same time.
When this technology matures, it will support backups to disk in yet another
way that tapes can never match. Coupled with a new breed of low-cost disk
drives, as well as storage array features that enable backups (snaps,
mirrors, clones, etc.), we're seeing the dawn of a new age of backup, IMHO.
tl
Terry Lemons
CLARiiON Applications Integration Engineering
EMC²
where information lives
4400 Computer Drive, MS D239
Westboro MA 01580
Phone: 508 898 7312
Email: Lemons_Terry AT emc DOT com
-----Original Message-----
From: Joe Moore [mailto:Joe.Moore AT sdrc DOT com]
Sent: Tuesday, December 10, 2002 4:32 PM
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: [Networker] DDS on file-type devices
Has anyone tried to share a file-type device with Dynamic Drive Sharing?
I was thinking that it should be possible to use a SAN-attached disk
with a cluster-aware filesystem (so that it can be mounted from multiple
systems) to have a file device on a storage node and a server at the
same time, and let DDS handle synchronization between the two.
Anyone seen this sort of config before?
--Joe
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