Re: Poor raid 1 performance?

In news:43946b71$0$61254$892e7fe2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,
Folkert Rienstra va escriure:
> "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?

I am sorry it appears I also was too terse...
No, I believe I understand what I wanted to say ;-) (another thing is the
meaning of what I actually wrote :-), but it was not the purpose of my
question above).

I was asking, since you wrote my sentence (describing just an alternative)
was "obviously not the way to do it", what should be "the way to do [RAID 1
data organization on disk]."


>>> 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.
[...]
> 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.

Disk commands are interesting to understand the pros and cons of an
interface implementation (and I agree I missed something here on the first
shot; I'd thank here all that provided me detailled explanations and
commentaries, by the way.)


However, only considering the interface will drive us into a perfect world
where the disk provides the datas to the controller and up without any
delay; and we all know this perfect world is not yet the one we live into.

So I was _also_ considering the physical organization of the sectors on the
media. And my idea is that the write operation on the RAID controller will
not do anything special (in other words, the RAID controller will issue the
same I/O command to both "disks" as what it received from the upper level,
including same sector numbers); while the read operation on the RAID
controller will be splitted as you explained (thanks.)

Now, when you look at a lower level ("disk" within quotes above), my idea is
that the write operation accesses the media without lost time for a
sequential run of sectors ; this in turn means that the read operation
(which, as seen by the disk, only is about the even- resp. odd-numbered
sectors, as passed inside the I/O commandes received).

And my (perhaps faulty) conclusion was that this represented a small penalty
(with respect to e.g. RAID 0, where the traffic of I/O commands while
reading sequencially is essentially the same), because the read operations
have to skip some sectors while accessing the media.


Antoine

.

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