Re: very strange (maybe) raid1 testing results

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On Wed, 30 May 2007, Jon Nelson wrote:

> On Thu, 31 May 2007, Richard Scobie wrote:
> 
> > Jon Nelson wrote:
> > 
> > > I am getting 70-80MB/s read rates as reported via dstat, and 60-80MB/s as
> > > reported by dd. What I don't understand is why just one disk is being used
> > > here, instead of two or more. I tried different versions of metadata, and
> > > using a bitmap makes no difference. I created the array with (allowing for
> > > variations of bitmap and metadata version):
> > 
> > This is normal for md RAID1. What you should find is that for 
> > concurrent reads, each read will be serviced by a different disk, 
> > until no. of reads = no. of drives.
> 
> Alright. To clarify, let's assume some process (like a single-threaded 
> webserver) using a raid1 to store content (who knows why, let's just say 
> it is), and also assume that the I/O load is 100% reads. Given that the 
> server does not fork (or create a thread) for each request, does that 
> mean that every single web request is essentially serviced from one 
> disk, always? What mechanism determines which disk actually services the 
> request?

It's probably bad form to reply to one's own posts, but I just found

static int read_balance(conf_t *conf, r1bio_t *r1_bio)

in raid1.c which, if I'm reading the rest of the source correctly, 
basically says "pick the disk whose current head position is closest". 
This *could* explain the behavior I was seeing. Is that not correct?

--
Jon Nelson <jnelson-linux-raid@xxxxxxxxxxx>
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