>
> On Fri, 19 Jun 1998, Davide Rossetti wrote:
> > Could someone explain to me why aren't they so exciting ?
> Well what excites me about RAID is the speed in random access. Different
> disks can seek to different places at the same time making me happy. It
so, you see a major inprovement in disk seek overlapping, don't you ?
> has taken the compiling bottleneck off my poor drives and put on my ppros
> where it should be. BTW for timing sequential access I simply
> do a:
>
> hdparm -t /dev/sda
>
> and compare that to
>
> hdparm -t /dev/md0
>
> It's almost x2 here.
here what I get:
Personalities : [2 raid0]
read_ahead 120 sectors
md0 : active raid0 sdb1 sde1 sdd1 sdf1 8193024 blocks 32k chunks
md1 : active raid0 sdb2 sde2 sdd2 sdf2 4112640 blocks 64k chunks
md2 : active raid0 sde3 sdf3 2731008 blocks 128k chunks
md3 : active raid0 sdb3 sdd3 2538240 blocks 64k chunks
[root@pcape6 iotests]# hdparm -t /dev/md0
/dev/md0:
Timing buffered disk reads: 16 MB in 1.56 seconds =10.26 MB/sec
[root@pcape6 iotests]# hdparm -t /dev/md1
/dev/md1:
Timing buffered disk reads: 16 MB in 2.08 seconds = 7.69 MB/sec
[root@pcape6 iotests]# hdparm -t /dev/md2
/dev/md2:
Timing buffered disk reads: 16 MB in 1.88 seconds = 8.51 MB/sec
[root@pcape6 iotests]# hdparm -t /dev/md3
/dev/md3:
Timing buffered disk reads: 16 MB in 2.06 seconds = 7.77 MB/sec
> >
> > It seems that the md driver is only remapping sectors, not using any
> > I/O overlapping. this way the performance gain is marginal.
> With sequential access the could very well be true. But then again what
> real life activity just makes a big file full of zeroes? :)
In real life those are not zeroes. they are data from/to our supercomputer
memories. And in our QCD(Quantum Cromo Dynamics) simulations, there are
HUGE amount of data to be read/written.
regards.
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|Rossetti Davide INFN - Sezione Roma I - gruppo V, prog. APEmille|
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