Eyal Lebedinsky wrote:
Nuno Silva wrote:
Hi,
Eyal Lebedinsky wrote:
I noticed that my 3-disk RAID was syncing at about 40MB/s, now that I
added a fourth disk it goes at only 20+MB/s. This is on an idle machine.
3*40=120
4*20=80
What does this mean? The raid is syncing at 20MB/s, not each disk, so I do
not see what the multiplication is about.
Yes, you're correct :-)
Individually, each disk measures 60+MB/s with hdparm.
And concurrent hdparms? Or some dd's concurrently?
I do not see this as relevant, but four concurrent hdparms (each to a
different disk) give about 30MB/s per disk. I expect the controller
to talk to the four disks at their full speed so concurrency should
not be the issue.
I guess you're using linux's software raid?
If so, you're hitting the 120MB/sec (and I *think* this time I'm
correct! :-)
If this is a PCI32/33mhz slot you're not going to be able to get more
juice. (I bet that 3 concurrent dd's gets you 40MB each).
Anyway, this may be offtopic because the problem (only 20MB/sec for the
raid with 4 disks) should be something else... Sorry for the noise.
kernel: 2.6.13 on ia32
Controller: Promise SATAII150 TX4
Disks: WD 320GB SATA
Q: Is this the way the raid code works? The way the disk-io is
managed? Or
could it be due to the SATA controller?
You can isolate the performance drop with some dd's. Maybe this card is
in a pci32/33mhz and you're hitting the pci bus' limits? (120~130MB/sec).
'hdparm -T' gives about 1250 MB/sec so this is not the limiting
factor.
Mine outputs some fabulous values too... I'm not sure I trust them ;)
# hdparm -T /dev/sda
/dev/sda:
Timing cached reads: 3536 MB in 2.00 seconds = 1767.38 MB/sec
Regards,
Nuno Silva
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