Oracle-L: RE: EMC question

From: Khedr, Waleed <Waleed.Khedr_at_FMR.COM>
Date: Tue, 02 Apr 2002 11:48:24 -0800
Message-ID: <F001.004390FF.20020402114824@fatcity.com>

HI Gaja & list,

I am trying to find the truth here! I was told by the storage group that EMC has striping now and this what I'm researching.

Gaja, what you described as a problem during WRITES to striped EMC is a typical procedure in any striped volumes.

There is a mid layer that has to map logical I/O addresses to physical disk addresses.

If you are using Veritas LVM then all reads/writes requests will be queued to the LVM and will do exactly as you mentioned.

There is a queue but the difference is big. You have a queue with many servers serving the queue requests simultaneously.

Please check powerlink.emc.com

Regards,

Waleed

-----Original Message-----
Sent: Tuesday, April 02, 2002 12:38 PM
To: Multiple recipients of list ORACLE-L

Waleed & list,

To define the terms we have on hand:-
A contiguous meta volume requires the hyper volumes to be sequential. A non-contiguous does not require the hyper volumes to be sequential.

I want to reiterate again that the concept of "pure striping" at the hardware level, is still not there in EMC, even though you have documentation that claims that you do. Let me explain.

When you look at "pure striping", there are 2 aspects to it :-

The read aspect
The write aspect

Take an example of a 4-way striped volume. The read aspect provides us the capability for all 4 drives to independently spin and service I/O from each of the drives. This the EMC device does, after the data has been placed on all hypers that support a meta volume.

The write aspect needs to offer the same functionality. So, if you are writing to 4 distinct blocks (each on 1 disk), then each drive should be able to write 1 block in an independent fashion.

That is where, the EMC hardware striping is not complete. This is because, the 4 blocks that need to be written to the "meta volume" with 4 hypers (regardless of whether it is contiguous or not), will happen in "sequential" fashion. Meaning, to write 4 blocks into the "striped volume", the first block will be written to the first hyper, followed by the second block to the second hyper and so on. As you can see the blocks that need to be written are queued up, so that they are written in a sequential fashion on the underlying hypers. This can and will cause severe write-intensive I/O bottlenecks.

Why is this implemented this way? No specific reasons, except, "that is how it is right now". It has been rumored that microcode 5x68 or 5x69, will do that. Remains to be seen.

So all the EMC "striping" does right now is to alleviate the problem of read-intensive operations not hammering a single drive, provided the data is spread across all the underlying hypers. I am not very familiar with the Engenuity product offering, hence cannot comment on that, but from what I have heard, it is a software-based volume management product.

Best regards,

Gaja

"Khedr, Waleed" <Waleed.Khedr_at_FMR.COM> wrote:
>
> It looks like it's available now.
>
> This is from: ResourcePak for Windows Version 3.2
> Product Guide
>
> "Symmetrix Microcode level 5x65 includes support for
> concatenated
> (contiguous) and striped metavolumes.
> Noncontiguous metavolumes (including striped)
> require EMC Enginuity(tm)
> (5x66 microcode) or higher."
>
> Regards,
>
> Waleed
> -----Original Message-----
> To: Multiple recipients of list ORACLE-L
> Sent: 4/1/02 5:38 PM
>
> Waleed & list,
>
> I researched this issue recently and found out that
> the meta volume was "concatenating a bunch of hyper
> volume". As you know, concatenation is NOT striping.
> The hyper volumes get filled with data one after
> another, eventually giving you the "simulation of a
> striped volume", when all hypers are filled with
> data.
>
> I don't know about the "single-host vs. multi-host"
> addressibility issue. There are plans for supporting
> "true striped volumes" in microcode level 68 or 69.
> From some "reliable sources" that does not look like
> it will happen any time soon. So until then, you
> should consider created mirrored hyper volumes
> within
> EMC (RAID 1) and then create a striped volume using
> Veritas Volume Manager, giving you a RAID 1+0
> configuration, which is ideal.
>
> Best regards,
>
> Gaja
>
> --- "Khedr, Waleed" <Waleed.Khedr_at_FMR.COM> wrote:
> > Four years ago the only hardware striping
> available
> > on EMC I was aware of
> > was RAID-S.
> > Recently researching striping ideas on EMC and was
> > told that we can achieve
> > raid0+1 hardware striping on EMC.
> >
> > I was told that EMC has some layer called
> > meta-volume that is made of many
> > other hyper-volumes.
> >
> > Told also that meta-volumes could be raid-0
> > (striped) and it's not a
> > single-host addressable volume.
> >
> > Does anybody know anything about this? Can we
> really
> > get a hardware striped
> > volume using EMC?
> >
> > Any limitations there?
> >
> > Thanks.
> >
> >
> > Waleed
> >
> >
> > Any views or opinions presented in this email are
> > solely those of the
> > author and do not necessarily represent those of
> the
> > company
> >
> > --

Gaja Krishna Vaidyanatha
Director, Storage Management Products,
Quest Software, Inc.
Co-author - Oracle Performance Tuning 101 http://www.osborne.com/database_erp/0072131454/0072131454.shtml

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Received on Tue Apr 02 2002 - 13:48:24 CST