multiplexing and tape-to-tape duplication

[spitman]

I hope this isn't a silly question...

I have jobs that back straight up to tape, and they have media multiplexing on in their schedule (set to 4). I was told this was done this way for time reasons--and I remember in the past year trying to turn that down to 1 and having the backup take forever.

A step backward--the reason they were sent straight to tape is because they were taking too long going straight to disk (again, at least that's what I was told).

If I set up an SLP that takes that tape backup and duplicates it to a second tape, will the second tape also be multiplexed? Would there be some way to have that second copy in a single stream?

The reason I ask is because restoring this multiplexed backup can take around 24 hours, which is excruciating... and I'm trying to lessen that pain.

Thanks in advance...

[Marianne]

I have seen in real life that restores from MPX tapes up to a value of 4 has no significant influence on restore speed.

If memory serves me right, the default for tape-to-tape duplication is for none-MPX. MPX setting must be explicitly specified/selected. (No access to a system right now to check SLP properties).
I have tried to duplicate without MPX in an attempt to ensure fast restores, but the duplication took way too long.

So, in my experience MPX up to 4 is good for backup and restore performance.

[RiaanBadenhorst · Accepted solution]

What doesn't make sense here is that someone directed them to tape because disk was slow. Its usually the other way around. Disk was introduced into backup to lighten the load on tape drives when it came to clients with low performance. You would back it up to disk at trickle speeds, then stream it off disk onto tape a high rate.

From what you say it does sound like you have a very slow client as the MPX 1 meant it took long. The benefit of MPX 4 is that you can squeeze 4x times (theoretically) the performance out of the client.

Any ways, duplication to tape would be NON MPX as it reads the image,and duplicates it, it doesn't ready the tape (by default).

-mpx

Specifies that when you duplicate multiplexed backups, NetBackup creates multiplexed backups on the destination media, which reduces the time to duplicate multiplexed backups.

Multiplexed duplication is not supported for the following operations:

• Non-multiplexed backups

• Backups from disk type storage units

• Backups to disk type storage units

• FlashBackup or NDMP backups

If backups in the previous categories are encountered during duplication, NetBackup duplicates them first and uses non-multiplexed duplication. It then duplicates the multiplexed backups by using multiplexed duplication.

If all the backups in a multiplexed group are not duplicated, the duplicated multiplexed group has a different fragment layout. (A multiplexed group is a set of backups that are multiplexed together during a single multiplexing session.)

If this option is not specified, all backups are duplicated by using non-multiplexed duplication.

[spitman]

Well, to combine your two responses, it sounds like:

1. Whether or not I use 1x multiplex or 4x multiplex, restore will take (relatively) the same time;

2. If multiplex makes backup faster, then the 8 hour backup I am seeing now takes 24 hours to restore, because of the fact that I'm backing up 4x streams and restoring at 1x stream at a time...

Does that make sense, or am I way off?

[watsons]

I second what Mariaane said about the 4x MPX setting, from my experience it is usually the optimal value if you need MPX. 2x is probably fine as well, 8x you will start to see restore taking very long. While all these are from our experiences, it may not be accurate on all env., so you will have to test in your own env. to get the best MPX value.

It is best you find out why disk backup is slower than tape backup - as Riaan pointed out this does not make sense, resolving this issue would put you in a better position to implement a better backup strategy in longer term.

For me, I try to avoid using MPX at all if I don't have to. RTO is quite important, a restore too long for me is probably less value as much.

[Marianne]

I agree with the need for restore testing. Test restores from multiplexed tapes and non-multiplexed duplicates.

I have seen more or less same restore as backup speed with MPX of 4. 
You need to see what works best in your environment.
There are other factors that will also influence restore speed - like mismatched NIC settings (which for some reason does not always cause issues with backups, but with restores).