There are some vendors that de-duplicate based on a sliding window out of a stream of data that can be adversely affected by multiplexing also. If you take a fixed block the statement I've gotten from the vendors is that if you mix streams of data using multiplexing the de-dupe ratio can decrease. Say for instance you are backing up 2 large databases and you run a full every day. If you back up the databases as separate streams you will get a great de-dupe ratio. See the below simple "diagram", each |--------| is a block.. |--------||--------||--------||--------||--------||--------| 111111111111111111111111111111111 (Database 1) |--------||--------||--------||--------||--------||--------| 222222222222222222222222222222222 (Database 2) Now, if you take those 2 databases and multiplex them together, like so: |--------||--------||--------||--------||--------||--------| 122122221111212122112212212121121 My blocks can be different now and I might not get the same de-dupe ratio. Granted, this is a very simple representation of it and I'm sure many people can pole many holes in this, but from the information I've gotten from the de-dupe vendors is that that multi-plexing can change the way the blocks are seen by the de-dupe engine and cause this type of inability to de-dupe. -Trey
________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Ed Wilts Sent: Wednesday, April 30, 2008 7:09 PM To: Mike Sparkes Cc: veritas-bu@mailman.eng.auburn.edu Subject: Re: [Veritas-bu] Multiplexing on VTLs On Wed, Apr 30, 2008 at 6:36 PM, Mike Sparkes <[EMAIL PROTECTED]> wrote: Multiplexing mixes streams of data from multiple sources into one stream to the storage device. A de-duplication product on that storage device will be breaking up the stream into blocks and looking for duplicate blocks. Let us assume that three backups are being multiplexed and that no data changed since the previous backup. It is unlikely that the data will mix together again at the same rate in the same ratios to create the same blocks and so they will be treated as unique and stored in full. I disagree. Multiplexing doesn't mix up the blocks coming from each server. You may see things like block 1 and 2 from server 1 followed by blocks 1, 2, and 3 from server 2, but if block 2 from server 1 is the same as block 3 from server 2, it will de-dupe. It doesn't matter what the speed is today versus yesterday - you're not de-duping the tape but you're de-duping the blocks. Duplication typically is not done by files - it's done by blocks, and that isn't changing with multiplexing. .../Ed Mike Sparkes ________________________________ From: Ed Wilts [mailto:[EMAIL PROTECTED] Sent: Wednesday, April 30, 2008 4:12 PM To: Mike Sparkes Cc: veritas-bu@mailman.eng.auburn.edu Subject: Re: [Veritas-bu] Multiplexing on VTLs On Wed, Apr 30, 2008 at 5:58 PM, Mike Sparkes <[EMAIL PROTECTED]> wrote: if you ever move to de-duplication, the act of multiplexing your backups ruins the ability to detect duplicate blocks. Your de-dupe ratio will be terrible. I don't follow your logic here. Why would multiplexing affect the de-dupe ratio? -- Ed Wilts, Mounds View, MN, USA RHCE, BCFP, BCSD, SCSP mailto:[EMAIL PROTECTED] If I've helped you, please make a donation to my favorite charity at http://firstgiving.com/edwilts
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