Re: Datafile recovery

Michael Sullivan <mps_at_panix.com> wrote in article <5l66ak$ic9_at_panix.com>...
> In article <33748975.80037682_at_allnews.nb.sympatico.ca>,
> Doug Meredith <no_at_junk.mail> wrote:
> >>Carefull with tar. It does compression and could lose some data eg
 repeating
> >>null values.
> >
> >Actually, tar doesn't do compression. Even if it did, I don't see why
> >there would be any risk of losing repeating null values.
> >
> >Don't worry about tar. It is tried and true.
>
> Agreed, tar is about the best at shuttling large files around, and even
> moving them from one file system, to another and between machines using
> remote shells.
>

 [SNIP]
>
> However, tar, as far as I know, along with standard compression
> utilities is a very safe way to go under most circumstances. The
> previous poster should re-check his sources or the person who informed
> him that tar has problems.
>
> Mike
> --
> Michael P. Sullivan PP-ASMEL/IA
> Aero-Vision Technologies, Inc. Skylane N52792
> E-Mail: msullivan_at_ACM.org or mps_at_aero-vision.com Phone: 516-952-3516
> * UNIX Systems and Database Consulting, Architecture and Management *
>

This is the situation as far as I understand it (and have been lead to believe by Oracle UK Support)

The problem is that some tape utilities ('pax' on AIX is a case in point, as is spax on secure unix platforms derived from AIX) don't save to tape/write back to disk any block that is all 0's. They just lseek and create a hole. Of course, when you read such a block off disk, Unix returns a blockfull of 0's. The when you write that block, is creates a new block in some non-contiguous bit of disk storage. It works, but you take a performance hit for ever after. Also, you can suddenly find that your database files that fitted into your disk partition suddenly don't fit anymore, thus leading to an instance shutdown with a write error, which is not good!

This is what I think the original poster meant by 'compression' (rather than gzip/pkzip/compress type compression).

Now, I wonder whether some Unixes just link tar to pax (pax can read/write almost every known tar and cpio format; very useful when getting files off an underdocumented tape). It could also be that some tars have the same feature. AFAIK, no version of cpio (especially when used with -c portability option, which is always good practice for disaster recovery) has this feature.

-- 
Miles Thomas
Logica UK Ltd
thomasm "at" logica "dot" com
The above are personal opinions, and are
not necessarily the opinions of my employer.