slow tar performance on fbsd5

[Tulio Guimarães da Silva]

Hi,
  I´ve got the same kind of problem, not only with DDS-[234] tapes, but 
also with "all-powerful-with-bells-and-whistles" AIT-3 units, with 
controllers ranging from Adaptec stock 2940 to PCI-X Ultra-320... almost 
same results.
  The problems seems to lie in tar itself; I read there´s something to 
do with block sizes, but using -b with larger values got me not much 
more than corrupt or incomplete data. :( The only way I got to have 
decent transfer rates AND reliability was to filter *archiving* through 
dd, including block sizes. For example, to archive:

  # tar -zcpf - /usr/local | dd of=/dev/sa0 bs=64k

and to restore:
  # tar -b 128 -zxvf /dev/sa0

  The above is particullarly true for remote transfers; if you´re using 
tar over rsh/rmt (-f host:/path), you´ll surely prefer simple "rsh/tar" 
with output redirection. ;)
  Note that block sizes in tar count as 512-byte ones, while in dd they 
can be specified as Kilobytes or even megabytes. Besides speed, there´s 
a sensible boost on storage space when using dd-block-sized transfers. 
The apparent reason for that is tar actually uses -b 20 (10kb) blocks, 
while 10GB+ tapes usually expect larger sizes.
  For AIT-3, I didn´t notice any real good improvement past 128kb block 
sizes; I didn´t experiment enough with DDS-4 because our test tape drive 
got a heart attack and quit... BTW, it returned 2 weeks ago and I didn´t 
give it any attention; it may be a little depressed by now, so I guess 
I´ll return it to test beds. :) Anyway, I wouldn´t try anything lower 
than 32kb blocks on it.
  I can´t remember if I did any test pointing to /dev/null, as mr. 
Hartland suggested, nor from /dev/zero or /dev/random... it´s worth a try.
  I´ll post new results as soon as I get´em (if any). ;)
  Have luck,

Tulio G. da Silva

Steven Hartland wrote:

> Might be silly but do u get similar results if u:
> 1. expand to a memory backed disk
> 2. expand to /dev/null
>
>    Steve
> ----- Original Message ----- From: "JG" <jarek at adeon.lublin.pl>
>
>> I had to unpack a lot of tar archives and I occasional noticed terrible
>> bad performance on freebsd5.
>