Two identical disks, different performance

webno · 04-08-2006, 11:39 AM

I'm building a server and I've put in two identical Hitachi 250GB SATA drives. I've set up my system (debian, xen, raid1 and lvm2) and noticed something strange when I started to tweak it.

/dev/sda reads perform bad while /dev/sdb reads perform normal.

Code:

raksuki:~# hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads:   38 MB in  3.22 seconds =  11.80 MB/sec

raksuki:~# hdparm -t /dev/sdb
/dev/sdb:
 Timing buffered disk reads:  172 MB in  3.02 seconds =  56.95 MB/sec

When I time de RAID1 device with

Code:

hdparm -t /dev/md0

then the performance alternates between sda's performance and sdb's performance which is probably logical. Both disks received the same treatments during the installation.
Anyone know what is going on?
Extra info: SuperMicro p4SCi, with a P4 3GHz and 1GB of memory running debian sarge and software RAID on ICH5R.

Brian1 · 04-08-2006, 06:14 PM

Might check the bios and make sure the sata channel is running in sata mode and not pata or something. Type ' hdparm -i /dev/sda ' and ' hdparm -i /dev/sdb ' and see if both using the same settings.

Brian1

webno · 04-10-2006, 10:48 AM

The hdparm -i are the same for both drives.
All dmesg output regarding both drives are equal. There is one line that might be of interest:

Code:

ICH5 not 100% native mode: will probe irqs later.

I played around in the BIOS and there is an option:

Serial ATA Port0 Mode-> SATA0 Master
Serial ATA Port1 Mode-> SATA1 Master

The first one can be set, and the second one follows automatically.
If I switch Port0 Mode to SATA1 Master, then the second drive becomes faster. So whichever drive is on serial ata port1 mode becomes the slower drive.

I pretty much tried all SATA modes and settings, switching between IDE and RAID or between enhanced mode, combined mode sata only mode but every time one drive underperforms.

..?

thermite_1033 · 04-11-2006, 04:46 AM

the 2 disk are running on the same controler, that way the master will be served first en the second will underperform

try to use the disks on different controllers

farslayer · 04-11-2006, 08:17 AM

SATA isn't supposed to have a master and slave situation like the old PATA drives..
if that's the cause then the Manufacturer (Intel) must have taken some shortcuts when making that controller chip.

I searched around a bit and found nothing on this problem.. very bizarre, and I'm really curious what the answer is.

webno · 04-11-2006, 02:31 PM

Good news, I found it. The Port0 Port1 thing basically switched the drives names. The second drive for example would only appear as /dev/sda. So it was the same drive that underperformed every time afterall. Next logical step was to replace the drive but that didn't help. And after some switching and testing I finaly found it: It was the SATA cable. Geez.

farslayer · 04-11-2006, 10:08 PM

Thanks for posting back the solution.. If I run across that problem in the future I'll swap out my cables before I go crazy chasing phantoms