On 20/04/2009 18:12, Johannes Segitz wrote:
i'm currently trying to create a raid 5 out of three 1 TB hdd. For now
there is one hdd missing so i get 3 TB of usable space.
[...]
Stride and stripe-width will be correct when i add another two hdd of
which one will carry data. Can someone please give me a hint why i
could get such bad performance especially while reading?
I would have thought it's because you're running in degraded mode and
one in 3 sectors is having to be regenerated from the parity. It still
seems a bit slow, though.
Here I have a 3-disc RAID-5 of similar drives:
# hdparm -i /dev/sda
/dev/sda:
Model=SAMSUNG HD103UJ , FwRev=1AA01112,
SerialNo=S1PVJ1CQ602164
Config={ Fixed }
RawCHS=16383/16/63, TrkSize=34902, SectSize=554, ECCbytes=4
BuffType=DualPortCache, BuffSize=32767kB, MaxMultSect=16, MultSect=?0?
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=268435455
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=yes: disabled (255) WriteCache=enabled
Drive conforms to: unknown: ATA/ATAPI-3 ATA/ATAPI-4 ATA/ATAPI-5
ATA/ATAPI-6 ATA/ATAPI-7
# mdadm --detail /dev/md1
/dev/md1:
Version : 00.90.03
Creation Time : Mon Jul 28 15:49:09 2008
Raid Level : raid5
Array Size : 1953310720 (1862.82 GiB 2000.19 GB)
Used Dev Size : 976655360 (931.41 GiB 1000.10 GB)
Raid Devices : 3
Total Devices : 3
Preferred Minor : 1
Persistence : Superblock is persistent
Intent Bitmap : Internal
Update Time : Tue Apr 21 00:35:26 2009
State : active
Active Devices : 3
Working Devices : 3
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 256K
UUID : d8c57a89:166ee722:23adec48:1574b5fc
Events : 0.6134
Number Major Minor RaidDevice State
0 8 2 0 active sync /dev/sda2
1 8 18 1 active sync /dev/sdb2
2 8 34 2 active sync /dev/sdc2
It has LVM and an ext3 filesystem on it. Here are my timings:
# time dd if=/dev/zero of=big_file bs=4096 count=2560000
2560000+0 records in
2560000+0 records out
10485760000 bytes (10 GB) copied, 264.448 seconds, 39.7 MB/s
real 4m25.740s
user 0m2.272s
sys 0m34.470s
# time dd if=big_file of=/dev/null bs=4096 count=2560000
2560000+0 records in
2560000+0 records out
10485760000 bytes (10 GB) copied, 53.9577 seconds, 194 MB/s
real 0m54.026s
user 0m0.556s
sys 0m4.944s
I'm not quite sure whether I should be disappointed at my writes being
so slow. Certainly there's a lot of rattling during writing, which
probably indicates lots of seeks to write ext3's journal. But reads are
roughly what I expected, at about three times the single-disc throughput.
Cheers,
John.
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