Re: Poor raid 1 performance?

In news:439db5c4$1$3384$892e7fe2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,
Folkert Rienstra va escriure:
> "Antoine Leca" <root@xxxxxxxxxxxxxxxxx> wrote in message
> news:439d7aa3$0$29626$636a15ce@xxxxxxxxxxxx
>> Or that the stripe size is lower than available free cache on the
>> RAID controller (and we hope so.)
>
> Cache has nothing got to do with it either except for when speaking of
> RAID on the same channel and long sequential reads.

Consider a single I/O command to read a number of sectors, say 2 stripes.
The RAID0 controller will issue two commands to each drive, each for a
single stripe. The drives will return the sectors, in order, _at the same
time_. The controller can pass down the sectors from the drive 0 directly to
the original poster, but it should buffer the sectors received from drive 1,
to pass them down _after_ all the sectors from drive 0 have been
transmitted.

And I only pointed out that if the memory on the controller is less than a
stripe, the situation is less clear.


>>> Read half of them, get half the throughput.
>>
>> My point, exactly.
>
> I doubt it. The comment was about RAID1 read in stripe fashion ...
>
>> Since there is two drives, we end up at twice this throughput when
>> data are delivered to the main system by the RAID controller, and
>> cannot make better while reading sequencially.
>
> ... where this comment is about RAID0.

No, it's about RAID1 (RAID0 can perform better than twice the
half-throughput of the drive, can't it?)


>> OTOH, RAID-0 may deliver an higher throughput, because it can use
>> more of the available bandwidth on the connection link.
>
> Nonsense. So can RAID1.

But you pointed out earlier that RAID1 was only reading half the sectors, so
delivered half the throughput...

What I do not know is if actual drives (with their internal caches) can
deliver the same throughput when you ask them to read say 10,000 continuous
sectors, as when you ask them to read all even-numbered sectors from 100,000
to 119,998 (count is 10,000 too). Former is typical of RAID-0, latter of
RAID-1.


> The interface is never a bottleneck, bandwidth wise, unless there is
> a deliberate interface mismatch between drives and host interface.

Yeah, I get that; I even believe there is design objective to have the
interface a few years in advance to drive technology, to allow the devices
implementing the interface still behaves "correctly" at the end of their
normal lifetime.

I seem to remember a time (around 1999) where the "usual" interface was at a
limit, and the newer drives then (ATA5-capable) were able to deliver more
bandwidth, so there was a waste; but I do not remember exactly where was the
bottleneck, ATA 40-wire or more plainly the PCI bus...


Antoine

.

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