Strange hard disk lag

Message

derekkraan · Post by **derekkraan** » Tue Apr 27, 2004 3:39 am

Ok, first off, this is very strange, and I have no idea what could be causing it, but here is the synopsis. All my hardware is right up to date, and I'm running kernel version 2.6.5. I've got two WD 80GB 7200RPM 8MB cache disks set up in a software RAID, which should be bloody fast, right?

Well, it's not really the speed that's bothering me. Whenever I call up a program(whether I'm root or not) there is a several second lag before I can detect any hard disk activity at all. When it finally does kick in there is a very brief time before the program is visible on my screen, and that part is very very quick.

The part I'm wondering about is the strange lag before the hard disk is read from. I've got reiser as my filesystem, but I can't see how that would affect it really.

Any help is very much appreciated. Thanks in advance!

pandaxiongmao · Post by **pandaxiongmao** » Tue Apr 27, 2004 3:53 am

Have you activated DMA?

derekkraan · Post by **derekkraan** » Tue Apr 27, 2004 4:07 am

Yes, I have enabled dma.

Rainmaker · Post by **Rainmaker** » Tue Apr 27, 2004 4:12 am

maybe useless, but can you post the output of hdparm -tT?

Can a few other people also post that (pref. with similair hardware). I'm not behind a linux box now, so I can't...

derekkraan · Post by **derekkraan** » Tue Apr 27, 2004 4:17 am

Sure can! Here's the output of testing the speeds of the two individual drives, and then the raid partition itself:

# hdparm -tT /dev/hde /dev/hdg /dev/md0

/dev/hde:
Timing buffer-cache reads: 1208 MB in 2.00 seconds = 602.89 MB/sec
Timing buffered disk reads: 132 MB in 3.04 seconds = 43.47 MB/sec

/dev/hdg:
Timing buffer-cache reads: 1208 MB in 2.00 seconds = 602.59 MB/sec
Timing buffered disk reads: 140 MB in 3.03 seconds = 46.27 MB/sec

/dev/md0:
Timing buffer-cache reads: 1196 MB in 2.01 seconds = 596.30 MB/sec
Timing buffered disk reads: 224 MB in 3.01 seconds = 74.45 MB/sec

Here's some other stuff hdparm has to say.

# hdparm /dev/hde /dev/hdg

/dev/hde:
multcount = 0 (off)
IO_support = 1 (32-bit)
unmaskirq = 1 (on)
using_dma = 1 (on)
keepsettings = 0 (off)
readonly = 0 (off)
readahead = 256 (on)
geometry = 16383/255/63, sectors = 156301488, start = 0

/dev/hdg:
multcount = 0 (off)
IO_support = 1 (32-bit)
unmaskirq = 1 (on)
using_dma = 1 (on)
keepsettings = 0 (off)
readonly = 0 (off)
readahead = 256 (on)
geometry = 16383/255/63, sectors = 156301488, start = 0

# hdparm -i /dev/hde /dev/hdg

/dev/hde:

Model=WDC WD800JB-00ETA0, FwRev=77.07W77, SerialNo=WD-WCAHL2570826
Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
RawCHS=16383/16/63, TrkSize=57600, SectSize=600, ECCbytes=74
BuffType=DualPortCache, BuffSize=8192kB, MaxMultSect=16, MultSect=off
CurCHS=4047/16/255, CurSects=16511760, LBA=yes, LBAsects=156301488
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5
AdvancedPM=no WriteCache=enabled
Drive conforms to: device does not report version:

* signifies the current active mode

/dev/hdg:

Model=WDC WD800JB-00ETA0, FwRev=77.07W77, SerialNo=WD-WCAHL2570878
Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
RawCHS=16383/16/63, TrkSize=57600, SectSize=600, ECCbytes=74
BuffType=DualPortCache, BuffSize=8192kB, MaxMultSect=16, MultSect=off
CurCHS=4047/16/255, CurSects=16511760, LBA=yes, LBAsects=156301488
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2
AdvancedPM=no WriteCache=enabled
Drive conforms to: device does not report version:

* signifies the current active mode

Rainmaker · Post by **Rainmaker** » Tue Apr 27, 2004 4:24 am

hmmm, try

hdparm -a8 /dev/hdX

your read-ahead is quite large (256 blocks), try setting it back to 8 blocks. This will be faster when handling smaller files...