Marianu, Jonathan wrote:
> My recollection is that during duplication from VTL to tape , it uses the mpx 
> originally set in the policy unless it is throttled down by the vault policy 
> but you can?t increase it. The MPX is what is so interesting to examine in 
> truss because I observed that any multiplexing will impact the duplication 
> speed. 
> 
> This is impact more pronounced though when you are mixing slow and fast 
> clients together because the fragments are not spread out evenly on the 
> virtual tape and bptm has to rewind the virtual tape and reread, causing a 
> delay to physical library which is torture on the physical tape drive 
> 
> Then I observed that a higher MPX led to a slight improvement in duplication 
> speed. I would use 24-32 
> However there are other tradeoffs. In the end we decided to not use VTL for 
> network client backups for the new master server environment model.  
> A problem with VTLis that a slow client backup will hold onto the vritual 
> media preventing me from using it for duplication. This impacts the RPO of 
> other clients backups. DSU does not have that issue. 
> We use 64TB EVA file systems per network media server which give pretty even 
> performance and we use vxfs. 
> One outstanding issue with file systems that you don?t have with VTL is 
> fragmentation Another benefit of VTL is that I have a media server dedicated 
> to writing the backups and another media server dedicated to duplicating 
> backups. 
> . 
> 
> __________________________________________
> Jonathan Marianu (mah ree ah' nu)
> AT&T Storage Planning and Design Architect
> (360) 597-6896
> Work Hours 0800-1800 PST M-F
> 
> Manager: David Anderson
> (314) 340-9296


Currently, when client backups are written to the VTL, we are using no 
multiplexing.  Regardless, our vault profile is set to de-multiplex backups 
when they are duplicated for faster restores from tape.  Are you saying to 
increase MPX to the VTL for client backups will improve vault duplication 
performance?  24-32MPX ?

To avoid the issue with backups tying up virtual tapes for duplication jobs, we 
reduced the cartridge size in the VTL to 100GB to have more tapes.  We rarely 
see vault waiting for a tape because it is in-use.

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