Re: Poor raid 1 performance?

"Antoine Leca" <root@xxxxxxxxxxxxxxxxx> wrote in message news:43940b8b$0$6485$636a15ce@xxxxxxxxxxxx
> In news:43938fba$0$44448$892e7fe2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,
> Folkert Rienstra va escriure:
> > "Antoine Leca" root@xxxxxxxxxxxxxxxxx> wrote in message news:439089e4$0$21227$626a54ce@xxxxxxxxxxxx
>
> > > With RAID 1, you cannot make them contiguous or near enough for
> > > both writing [every sector] and reading [every two sectors].
> >
> > So that's obviously not the way to do it.
>
> Sorry, you are too terse for me. Which "way" are you writing about?

You ask me to explain your own writing?

>
>
> > > So the obvious organisation is to make OK for writing (so just like
> > > a normal disk), and while sequential reading you should wait while
> > > skipping the odd-numbered sector which is in between.
> >
> > That's silly.
>
> May I ask you how it should be done then?

No. You can read the paragraph below instead.

>
>
> > You divide a single request into two and send each half to a
> > different drive. With consequtive requests (for a sequential file)
> > you sent half the number of total requests to one drive and the
> > other half to the other one, resulting in both drives reading
> > sequentially.
>
> Sure, sorry I had assumed this explanation.

Uh, what?

> And since the media have been written sequentially (I assume it, but I'd
> happy to learn a better way), if you are reading all the even-numbered
> (resp. odd-numbered) sectors, so the disk controler have to wait for the
> media a small bit between each sector to be transferred,

You need to get rid of that even/odd numbered sectors fascination and start
thinking in disk IO-commands, the way I described in the previous paragraph.

> which should give a small penalty

Nope, that's a big penalty.
Reading half the sectors per time unit obviously get's you half the
throughput. Combined with the other drive you will get your 100% back.

> with respect to a non-RAID configuration
> (assuming media access is the bottleneck, that is).
>
>
> > > There is a gain though (half traffic on the wire, for example),
> >
> > That's not a perceptive gain.
>
> /Quite/ possible ;-).
>
>
> Antoine
.

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