[CentOS] Performance Issues

Sun Sep 2 16:23:48 UTC 2007
Les Mikesell <lesmikesell at gmail.com>

Dan Dansereau wrote:
> My apology for cross posting
> We have a DELL6850 with 8Gbytes of memory,  four 3.2Ghz CPU's , perc 4
> raid controller, with fourteen 300Gbyte 10Krpm disk on a powervault
> 220s, And a powervault 124T  LTO-3 tape systems on a separate
> 160Mbyte/sec adaptec SCSI card.
> The disks are configured as two 2Tbyte raid 0 partitions using the perc
> 4 hardware. 
> The problem is - reading from the disk, and writing to the tape is
> pathetically slow.
> The specs say I should get around 80Mbyte/second - in reality - is about
> 500 Kbytes/second 
> Writing just to tape, using /dev/zero as input and dd to write, I can
> get up to 60Mybte/second, indicating that the tape drive and scsi card
> is functional from a hardware perspective. Since I can read and write
> data to the disks - I assume that they are functional.
> The files are 80Mybtes for the minimum size that I am writing to tape. 
> DELL Support hides behind the fact that CENTOS is not a supported OS,
> But RED HAT is - so switch to it, so DELL can get RED HAT Support
> involved. And the fact that Rocks is a modification to an unsupported
> OS.... 
> I have tried using various block size in tar from 256 to 8192 - with
> minimal success.
> Does anybody have any experience on setting the OS/Kernel parameters to
> utilize the hardware at something more then a snails pace?
> Or
> Suggestions on what I could test and/or try?

I don't know anything about this specific hardware, but the usual 
problem with tape is that if you can't feed it fast enough to stream at 
its native speed it has to constantly stop, back up, and restart to 
write the next chunk.  Single-threaded programs normally can't because 
they wait for a read to complete before starting the write.  There are 
programs around like mbuffer that allow the reading and writing to 
overlap and provide a large buffer to minimize the restarts even if the 
average throughput is still too slow.

Tapes also have a 'block size' that you can set with mt, and some 
particular size may provide the best speed.  With some drives, the write 
block size from tar or obs= in dd must exactly match the tape block size 
  or be exact multiples.

   Les Mikesell
    lesmikesell at gmail.com