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Old 06-06-2003, 12:43 AM   #1
anoop_chandran
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Registered: Nov 2001
Distribution: Redhat 7.0 ,mandrake 8.0 ,Redhat 7.2
Posts: 99

Rep: Reputation: 15
raid performance issue


hi,

i have following settings on a redhat linux 8.0 for raid 5 configuration
/etc/raidtab contains:

raiddev /dev/md0
raid-level 5
nr-raid-disks 2
nr-spare-disks 0
persistent-superblock 1
chunk-size 64k
parity-algorithm left-symmetric

device /dev/hda14
raid-disk 0
device /dev/hda20
raid-disk 1
---------------------------------------------------------------------------------------------------------
i have mounted /dev/md0 as /raid and can read and write to it ..
cat mdstat gives this o/p.

# cat //proc/mdstat
Personalities : [raid5]
read_ahead 1024 sectors
md0 : active raid5 hda20[1] hda14[0]
1020032 blocks level 5, 64k chunk, algorithm 2 [2/2] [UU]

unused devices: <none>
---------------------------------------------------------------------------------------------------------
the hard disk i have is just IDE and i use hda14 and hda20 for "raiding"
hdparm -i gives following o/p


# hdparm -i /dev/hda

/dev/hda:

Model=ST340810A, FwRev=3.60, SerialNo=3GC0ZFFM
Config={ HardSect NotMFM HdSw>15uSec Fixed DTR>10Mbs RotSpdTol>.5% }
RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=4
BuffType=unknown, BuffSize=2048kB, MaxMultSect=16, MultSect=16
CurCHS=4047/16/255, CurSects=16511760, LBA=yes, LBAsects=78165360
IORDY=on/off, tPIO={min:240,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=yes: unknown setting WriteCache=enabled
Drive conforms to: device does not report version: 1 2 3 4 5 6
---------------------------------------------------------------------------------------------------------
hdparm for bufferd disk/cache reads on hda14 or any other partitions givs me a value
close to 22-25 MB/sec (disk read ) and 85-94 MB/Sec (cache read)

# hdparm -t /dev/hda14

/dev/hda14:
Timing buffered disk reads: 64 MB in 2.87 seconds = 22.29 MB/sec

# hdparm -T /dev/hda14

/dev/hda14:
Timing buffer-cache reads: 128 MB in 1.50 seconds = 85.11 MB/sec

---------------------------------------------------------------------------------------------------------
but when i use hdparm on /dev/md0 ( my raid5 disk) i get drastically low read performance
i get around only 6 - 8 MB/Sec (disk read).
okay hdparm has to read from 2 partitions and there is
the parity calculation going on etc. .. but shouldn't i be getting a bit more higher disk reads around (10-12 MB/Sec)..

or is this quite normal.. ??

# hdparm -t /dev/md0

/dev/md0:
Timing buffered disk reads: 64 MB in 8.12 seconds = 7.88 MB/sec

# hdparm -T /dev/md0

/dev/md0:
Timing buffer-cache reads: 128 MB in 1.51 seconds = 84.78 MB/sec
---------------------------------------------------------------------------------------------------------
is there any way to get higher performance from my raid config..
i have the following CPU model

model name : Pentium III (Coppermine)
stepping : 3
cpu MHz : 601.345
cache size : 256 KB

hda size -> 40 gb
partitions from hda1 through hda20

filesystem on /dev/md0 is reiserfs...

i am planning to use the device as my squid - cache
thanks in advance
anoop.

Last edited by anoop_chandran; 06-06-2003 at 01:14 AM.
 
Old 06-06-2003, 05:04 AM   #2
Vlad_M
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Registered: Aug 2002
Location: Cape Town, South Africa
Distribution: Red Hat 8.0 (Home), Red Hat 8.0 (Work)
Posts: 388

Rep: Reputation: 30
How much research did you do on raid? My guess is NONE.

Here are the mistakes you have made:

-raiding twp partitions of the SAME disk
-trying to set up raid 5 with only two disks (which are actually 2 partitions of the same disk)


Basically you know nothing about raid, and you have only heard that it will work faster and be more reliable, right?

OK, now that I have gotten that off my chest, here is the helpful bit. Go to www.tldp.org and search for the linux software raid HOWTO. It will, amongst other things, explain to you exactly what raid is and is NOT abaout, and it will guide you step by step through the setup process, and even the testing process.

and if you have any problems after reading that, post here and we'll sort them out quick sticks!!!

Cheers mate!

Last edited by Vlad_M; 06-06-2003 at 05:05 AM.
 
  


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