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Advise for Differential tape

Wajeeh
Level 4

Dear All,

I have 8/9 virtual servers in my environment for which I am capturing image backup using agent for vmware from symantec. BE 15 with FP5 I am using.

I take full image backup of VM servers on week end and on daily I take a differential backup for all those. For purpose of differential I am using slot 1 in my autoloader and only one tape, daily new differential backup is appended to it while expiry of previous differential is set to 2 days. My concern is if this is ok to use one differential tape OR should be like one day differential job to be written on one tape next day on another one.

 

please advise, also if my question is not clear, let me know.

regards,

7 REPLIES 7

pkh
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It does not matter either way. Just make sure that the tape with the latest diff backup and the tape with the full backup are online when you do a restore

kf2013
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How many does your autoloader have? For safety, the next differential job should be write to another slot.

Be reminded that if the differential job would fall back to full backup, then one tape is not enough for it, so you should reserve more slots.

Hello,

I did not get "Be reminded that if the differential job would fall back to full backup, then one tape is not enough for it, so you should reserve more slots."

I have 8 slot in total, one is cleaning tape, 2 and 3 are used for full backup tapes while slot one is having one differential tape which is used daily.

Colin_Weaver
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The coment about the differential falling back to a full is just an infromational comment from that poster that if you are experiencing the condition where the differential is not possible, then the differential job (running as a full) will use more space than expected and might therefor cause a media insert request if there is enough space for a differential to complete but not a full.

You jobs logs would show if a differential has run as a full.

 

Your problem itself still realets to what is avaiable (in the partition if using partitions or in the library if not) in terms of appendable availability, overwritable availability and if a tape is sucessfully chosen, free space on the tape.

Colin_Weaver
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Just realized something I missed earlier

 

You have set the oberwrite prorection of your differential set to 2 days. Have you done this on the assumption that the first set on the tape can be overwritten after 2 days ?

 

Basically this is an invalid assumption if you are as the overwrite protection is based on the tape (and the last write to the tape.) and not on individual sets on the tape

 

Let's say you do the first differential of the week on a Monday and this differential does start with an overwrite (new tape, or tape is not appendable but is overwritable) The 2 day setting will means that the set in question will be overwritable on Wednesday against a time that matches the end of the Monday backup.

You then run Tuesday's Differential backup to the same tape as an append and the tape has enough space. At the end of Tueday's job the overwrite protection of the tape will move back to Thursday with this protection now affacting Monday's backup set a and Tuesday's.

On Wednesday, Monday, Tuesday and Wednesday's sets will be protected until Friday as such but the time that last diffenetial of the week runs On Friday - Monday, Tuesday, Wednesday, Thursday and Friday's sets will all be overwrite protected until Sunday night

Now this may not be a problem if your differential sets for the week will fit on one tape (bear in mind each differential is larger than the previous one) but will give an issue if tthe 5 days won't all fit on one tape

Now with regards good practice

Your full backup is on a different tape but if something happens to damage the differential tape (let's say on the Thursday with the ful on a Friday) can you afford to lose a whole week's worth of data if at the same times  as the tape being damaged you also lose some regularly changing production data - if you can accept this this then no issue with all the differentials on one tape however it would probably be better to use the same tape on Monday and Wednesday and then a different tape on Tuesday and Thursday with the full on the Friday (on a 3rd tape) as then you mininize the impact of loss of one differential tape.

Of course that example miminizes the risk whilst not using one tape per differential so is a balance of tape cost and management vs the impact of a single tape loss.

Taking this one step further however - your full backups from consecutive weeks should be on different tapes and you might therefore want to keep the differential sets for 9 days (1 week plus 2 days) so that then you always have a set of differentials to add to the previous  full. Again this would use more tapes but really the descision should be based on your intended backup and restore strategy against what the business can afford to have as downtime/loss of data if the worse case scenarios hapen.  Usually this cost of a few more tapes is a lot less than what the data is worth to the business.

 

 

Hello Colin, Sorry for very late response:

"On Wednesday, Monday, Tuesday and Wednesday's sets wil be profceted until Friday as such but the time tha lsst diffenetial of the weke runs On Friday - Monday, Tuesday, Wednesay, Thursday and Friday's sets will all be overwrite protected until Sunday night Now this may not be a problem if your differential sets for the week will fit on one tape (bear in mind each differentialis larger than the previous one) but wi guve an issue if tthe 5 days won't all firt on one tape"

Yes, in our case differential jobs fit ok in one tape, since have few virtual machines. ok for good practice you are advising to keep two differential tapes, so I change tape daily in differential slot? because I have one slot defined for differential OR I create a new partition in library with two slot for differential, in this case how I tell backup exec to write one day differential to one tape and another day differential to other tape?

Yes, our full backups for consecutive weeks are on different tapes

 

Colin_Weaver
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Employee Accredited Certified

Unfortunately there is no one best ptactice for this - we can tell you the effects of different scenarios but only you (or perhaps the owners of the data you backup) can decide whether or not what you implement is good for you