Re: raid5 read performance

["JaniD++" <djani22@dynamicweb.hu>]

----- Original Message ----- 
From: "Raz Ben-Jehuda(caro)" <raziebe@gmail.com>
To: "JaniD++" <djani22@dynamicweb.hu>
Cc: "Linux RAID Mailing List" <linux-raid@vger.kernel.org>
Sent: Tuesday, January 10, 2006 12:25 AM
Subject: Re: raid5 read performance


> 1. it is not good to use so many disks in one raid. this means that in
> degraded mode
>     10 disks would be needed to reconstruct one slice of data.
> 2. i did not understand what is raid purpose.

Yes, i know that.
In my system, this was the best choise.

I have 4 disk node inside 4x12 Maxtor 200GB (exactly 10xIDE+2xSATA).
The disk nodes sevres nbd.
The concentrator joins the nodes with sw-raid0

The system is a generally free web storage.

> 3. 10 MB/s is very slow. what sort of disks do u have ?

4x(2xSATA+10xIDE) Maxtor 200GB

The system sometimes have 500-800-1000 downloaders at same time.
In this load, the per node traffic is only 10MB/s. (~100Mbit/s)

First i think the sync/async IO problem.
At this time i think the bottleneck on the nodes is the PCI-32 with 8 HDD.
:(

> 4. what is the raid stripe size ?

Currently all raid layers have 32KB chunks.

Cheers,
Janos

>
> On 1/4/06, JaniD++ <djani22@dynamicweb.hu> wrote:
> >
> > ----- Original Message -----
> > From: "Raz Ben-Jehuda(caro)" <raziebe@gmail.com>
> > To: "JaniD++" <djani22@dynamicweb.hu>
> > Cc: "Linux RAID Mailing List" <linux-raid@vger.kernel.org>
> > Sent: Wednesday, January 04, 2006 2:49 PM
> > Subject: Re: raid5 read performance
> >
> >
> > > 1. do you want the code ?
> >
> > Yes.
> > If it is difficult to set.
> > I use 4 big raid5 array (4 disk node), and the performace is not too
good.
> > My standalone disk can do ~50MB/s, but 11 disk in one raid array does
only
> > ~150Mbit/s.
> > (With linear read using dd)
> > At this time i think this is my systems pci-bus bottleneck.
> > But on normal use, and random seeks, i am happy, if one disk-node can do
> > 10MB/s ! :-(
> >
> > Thats why i am guessing this...
> >
> > > 2. I managed to gain linear perfromance with raid5.
> > >     it seems that both raid 5 and raid 0 are caching read a head
buffers.
> > >     raid 5 cached small amount of read a head while raid0 did not.
> >
> > Aham.
> > But...
> > I dont understand...
> > You wrote that, the RAID5 is slower than RAID0.
> > The read a head buffering/caching is bad for performance?
> >
> > Cheers,
> > Janos
> >
> >
> > >
> > >
> > > On 1/4/06, JaniD++ <djani22@dynamicweb.hu> wrote:
> > > >
> > > > ----- Original Message -----
> > > > From: "Raz Ben-Jehuda(caro)" <raziebe@gmail.com>
> > > > To: "Mark Hahn" <hahn@physics.mcmaster.ca>
> > > > Cc: "Linux RAID Mailing List" <linux-raid@vger.kernel.org>
> > > > Sent: Wednesday, January 04, 2006 9:14 AM
> > > > Subject: Re: raid5 read performance
> > > >
> > > >
> > > > > I guess i was not clear enough.
> > > > >
> > > > > i am using raid5 over 3 maxtor disks. the chunk size is 1MB.
> > > > > i mesured the io coming from one disk alone when I READ
> > > > > from it with 1MB buffers , and i know that it is ~32MB/s.
> > > > >
> > > > > I created raid0 over two disks and my throughput grown to
> > > > > 64 MB/s.
> > > > >
> > > > > Doing the same thing with raid5 ended in 32 MB/s.
> > > > >
> > > > > I am using async io since i do not want to wait for several disks
> > > > > when i send an IO. By sending a buffer which is striped aligned
> > > > > i am supposed to have one to one relation between a disk and an
> > > > > io.
> > > > >
> > > > > iostat show that all of the three disks work but not fully.
> > > >
> > > > Hello,
> > > >
> > > > How do you set sync/async io?
> > > > Please, let me know! :-)
> > > >
> > > > Thanks,
> > > > Janos
> > > >
> > > >
> > > >
> > >
> > >
> > > --
> > > Raz
> >
> >
>
>
> --
> Raz
> -
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