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Operating System - Tru64 Unix
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vdump compression not necessary?

Derk Korevaar
Occasional Advisor

vdump compression not necessary?

Hi,

On our site, all our backups are made to tape (TZ88 and TZ89) with vdump -C. Does it make sense to specify -C as the tape drives do compression by default.

Or so I assume, is there a way to specify hardware compression other then using the button on the drive?

Thanks for any help,

Derk Korevaar
9 REPLIES
Mobeen_1
Esteemed Contributor

Re: vdump compression not necessary?

Derk,
From what i understand hardware compression and software compression work differently. You could complement these two together.

Hardware compression is normally done by the drive and is built on the drive. You should be able to turn this on or off. As to how you do this, it depends on the type of drive, library, software etc that you are using

Yes, it perfectly makes sense to go in for hardware compression even after software compression as you will surely gain some additional reduction, how much? this really depends on lot of other stuff as well

regards
Mobeen
Michael Schulte zur Sur
Honored Contributor

Re: vdump compression not necessary?

Hi,

you must consider:
How much space do I need on tape?
How busy is my machine?

If your machine is busy, it might be helpful to save the cpu from doing the compresseion stuff although I do not have much experience how much that will be.

hth,

Michael
Victor Semaska
Frequent Advisor

Re: vdump compression not necessary?

Derk,

From my experience always use hardware compression if it's available. Software compression tends to be CPU intensive. If your system is busy during backups, they'll take a lot longer to complete.

I disagree with Mobeen and believe that you shouldn't use hardware and software compression together.

If you're using Tru64 V5.x refer to the manpage 'tape' for further details. In my case I always use /dev/ntape/tape0_d1 to enable hardware compression. The /ntape means no automatic rewind after done, in case you want to put more than one backup on a tape. On Tru64 V4.0x I used /dev/nrmt0h.

Vic
Brian_254
Occasional Visitor

Re: vdump compression not necessary?

I've done some tests and found that hardware compression slows down back ups (TL891) as the tape drive takes longer to write.

With software compression, the i/o to tape is much quicker, but there is a CPU cost on the host doing compression.

So when deciding whether to use hw or sw compression, it depends what is more important, speed or cpu?

...also agree that using both is unecessary
Hein van den Heuvel
Honored Contributor

Re: vdump compression not necessary?

Hi Derk,

Other possible factor which would make me lean towards OS based compression are
1- The connection from host to tapedrive.
2- The 'portability' of the tape.
3- Backups are done during lower usage periods?

1- If you postpone compression for the hardware, then all the raw bytes will have to be pushed out over the cable/fibre/network. There might be a bandwith limit there.

2- Not sure this is an important argument, but is hardware compression portable? Can you read it with an other vendors drive? I honestly do not know, but it seems a relevant question.

3- Sure, host compression eats CPU, but it probably will not increase the elapsed time much/any with IO/CPU overlap (read ahead caches in the file system). And is the backup not done in a relatively low cpu-usage time window?


I do not expect hardware compression to save significant on top of software compression. Yeah the rules are different and more savings could be made, but I doubt it is any more than say 2%. Compare to zipping up JPG files. Not much to be gained there is there?

Groetjes,
Hein.
Orrin
Valued Contributor

Re: vdump compression not necessary?

Hi Derk,
I agree with all the comments given so far.

The only reason you would avoid software compression is, when you have a heavy reporting run which coincides with the backup window. if both run, something will have to give, and you don't want to chance it.

With hardware compression, your only concern is the bandwidth to the tape, which in most cases if inadequate will mean the backup will be slower.

As far as specifying hardware compression goes, if you issue a man command on tape, scroll down to the section where TZ88 and TZ89 are mentioned. In these areas you will find the devices and names mentioned with the different compression values.

As mentioned by a forum member before
/dev/ntape/tapex_d1 is for maximum compression.

Hope this helps..
Regards,
Orrin.
Mobeen_1
Esteemed Contributor

Re: vdump compression not necessary?

Victor,
I don't recommend that one should do both hardware and software compression together. This is a whole new subject, to be able to decide you should look at

1. Type of host

2. Type of data

3. Amount of data

4. Lot of other factors

Only after reviewing through all and may be some additional terms, a meaning full decision could be made.

We have been using for some of the above reasons mentioned above for some of our backup clients compression at h/w and s/w levels and have been doing this past couple of years and it works fine.

As i said, we don't do this for other clients for some of the reasons mentioned.

regards
Mobeen
Yong_7
Frequent Advisor

Re: vdump compression not necessary?

Hi Derk,

is this you are looking for ?

****************start **************
Device Special Files and Tape Density

Using a particular device special file with a certain tape format will
indicate what density code, compression, capacity and I/O is supported. It
is also possible for administrators to customize which device special file
names apply to what densities. Future extensions to device special files
may also increase the number of alternative files that can be used with a
particular device. The current range of possible device special files is:
tape?_d0 to tape?_d7, where ? is a decimal number representing the identif-
ication number of the tape device. Normally, the _d1 suffix will always
indicate the highest density used with compression, while the _d2 suffix
will indicate the highest density witout compression.

************** end **************

"#man tape " give your more details.

hth.

YJ

Derk Korevaar
Occasional Advisor

Re: vdump compression not necessary?

Hi,

Thank you for your response. I discovered that for V4 it is 'man mtio' but the extensions (h, m, l) remain the same. The man page is silent about the combination of h (high density, compression) and something like vdump -C.

Thanks,

Derk