PowerEdge, CentOS and LTO2: Tape is extremely slow 2 to 4 MB/s

[Kuba Ober]

On Friday 04 May 2007, Weichmann, Patrick wrote:
> Hi
>
> We have just taken live a site with 2 PowerEdge 2900 and we experience
> extremly slow backup speeds to LTO2 Tape. We need to save about 160 GB
> of Data but at a speed rate of 2 - 4 MB/s it takes ages.
>
> Does anyone use a LTO2 and what are their experiences? We installed
> CentOS5 and are using cpio.

First of all, don't use cpio nor tar. The filesystem specific dump utilities 
aren't all that great either. Use star, which handles extended attributes and 
ACLs just fine across the board (does for me, at least) 
http://cdrecord.berlios.de/old/private/star.html

When doing the backup, make sure that the tape streams all the time (i.e. it 
only reverses when changing tracks). If what you hear is tape going back and 
forth, you may need extra buffering, and for that you'll have to write a 
small C program. dd doesn't do what you think it does in that respect (don't 
use it).

I've had much success with simply mlock-ing a large buffer (64MB is a good 
figure to start with), outputting from it to the drive when it's 90% full, 
and stopping output when it's empty. That way the tape never looses 
streaming, even when lots of tiny files are being backed up.

Cheers, Kuba