Thanks to all who tried answering either part or all
of my question, Jeff Beck, Peyton Bland, Pat Obrien
and Alan Davis.
Thanks also to Alan_at_nabeth, who answered it in its
entirety as shown below.
>From this, I conclude (there was no tech objection so
far), vdump method is appropriate to determine what
compression ratio I get for my files, when using the
compressed device file. And in my tests, since most of
my files are binary, I loose in terms of time (takes
longer to compress and later restore) and money (uses
more tape). The vdump I posted in original question
was not actually dumping to tape, but was rather just
simulating, piped thru compress to get the actual
compression rate.
1. Odds are the two algorithms are sufficiently
similar that you won't increase the compression.
2. True.
3. The performance of backup programs is very
sensitive to the organization of the file system. A
very large file system with just a few large files,
can run very quickly. A file system with a lot of
small files, reasonably distributed, still has the
overhead of opening and closing lots of files. The
closing part
will generate a write load one way or another as the
access or inode modification times are updated. A
file system with lots of small files poorly organized
may trigger than less desirable search characteristics
of the file system's lookup code.
Depending on where the bottleneck is, there may not be
anything you can do.
4. The basic open, read, close sequence of vdump and
tar are likely to be much the same. If there is a
significant difference in the performance of the two,
it could be a fixable problem in the one. Any backup
tool that works through the file system is going to
have this basic open, read, close sequence that could
be a problem on some file-systems.
Other viewpoints:
Don't forget that the data rate to the tape can have
large effects on backup time and amount of data stored
on tape. If the system can't keep the tape i/o buffers
full it either has to start/stop or waste tape while
it
waits for more data to write. If the disks and tape
are on the same bus it can seriously degrade
performance due to bus contention.
*************
I'm not sure what the data rate for your tape drive
is. I use a tl891 (essentially a tz89 instead of a
tz88) and I come close to driving it at it's rated
5MB/sec, which works out to about 4 minutes/GB.
Taking your 315GB times 4 minutes (1260) and dividing
by 60 minutes/hour gives a time of 21 hours. If your
tape drive can only handle 2.5MB/s, you'd be looking
at around 42 hours, so your 80% done after 36 hours is
probably within range of what your tape drive can
handle.
**************************
--- Tru64 User <tru64user_at_yahoo.com> wrote:
> Hi,
> Trying to determine the true compression ratio of
> our
> data (advfs) to tape, so used:
> vdump 0f - /filesystem|compress -v|wc -c
>
> For one file system, (binary files most of them), it
> gave me -23%compression, thus 25GB of data, upon
> compression, spitted out 31GB. (I know about how
> some
> other files enlarge when compressed)
> Questions:
> 1. Using device nrmt0h (4.0g, with a tz88 20/40
> drive), is it clear that using compression will
> actually just take more tapes(dltIV) and time? or
> does
> the algorithm for compress I used with vdump differ
> with that of the device file?
> 2. If #1 is clear/true, then it is also true that
> there is no way to fit 40GB of data into this tape
> (DLTIV) using this drive.
> 3. Started a vdump (as above) on a 315GB fs monday,
> and now it is 36hrs later and it is still going
> 80%done, is that normal or is there something I
> could
> have done to speed up the process (4.0G, 2100Alpha,
> 4CPU's EV5, and, and, mmmhhh!! , 128GB swap too
> small
> huh?...non production machine)
> 4.I usually dont use vdump. I use plain old tar for
> bkups. Would a real bkup to tape of a filesystem,
> 315GB take this long using vdump?
>
> _Thanks in advance.
>
> Richard
> SAIC
> Greenbelt, MD
>
> =====
>
>
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Received on Wed Aug 01 2001 - 18:36:40 NZST