Re: Performance Question: RAID or Individual drives ?

Dominic Baines wrote in message <35AC7D22.59B834B6_at_hermes.cam.ac.uk>...
>Yes but it would be great if I could schedule scripts on a 24 basis
completely
>unattended just to see wouldn't it ?
>
>> You could try using - on UNIX - a shell script to log the output from
iostat,
>> vmstat, sar, etc.
>
>Yes, but it NT :-( (unfortunately)

Look at the MKS toolkit for NT. People who use it highly rate it. There's a demo on their site, I believe.

>
>> Or (and on NT) buy some additional software - although I've found
>> this can be pretty expensive.
>
>Thought some interrogation of sysdate both before and after and calculating
the
>difference may be acceptable but then that wouldn't give me a real picture
of how long
>the process took or would it ?
>

To be honest, I do this all the time. As long as the system is in a known state - i.e. nothing else is running on it - then your results will be accurate. I tend to run fairly long tests to overcome any 'funnies' like a bit of intermitant paging. I also run the tests multiple times and aggregate the results to overcome the effects of Oracle and OS buffers.

Again I tend to log the disk and CPU statistics, so you can spot 'strange' results quite easily (i.e. someone jumping in a running a big job in the middle of your tests!)

>> > At the end of the day I suppose it is the client and SLA's that will
win on a
>> > recovery arguament rather than performance and as we both answer to the
same
>> > person they will have to make a choice. The database is approximately
50/50
>> > read/write and it is growing at the rate almost across every tablepsace
of some
>> > 15% per year in size.
>>
>> Yep, many of my clients will happily accept the performance hit of RAID5
for the
>> better fault-tolerance and lower cost, over, say, RAID0+1.
>
>RAID is great for normal filesystems and it sure helps for disater recovery
but it
>isn't exactly kind on performance is it now?

Well on READ, most of the results I've seen and found myself show it not to be too bad. On my latest kit, RAID5 is just as quick as striped disks. On WRITE, the impact can be massive, maybe a factor of 2 or 3. That's high. But when you have clients like I do who run 1TB sized systems with datawarehouses (i.e., mainly read) then RAID5 is much, much cheaper.

>And, there is always one who will say that
>"....when's the last time we had to recover from drive failure? Isn't that
what the
>backup's are for ? ....such and such takes too long... what are you going
to do to make
>it faster ...." killing RAID and implimenting decent back-ups and DR would
>work......... or would it ?

Depends on your requirements. I suggested to my current client that they implement a hot-spare disk on a RAID5 system they'd already set-up. The Oracle db on it was heavily used and mission critical. They bitched for a while over the cost of 1 disk (!), but finally agreed.

6 months later - BOOM! We lost a disk. The system rebuilt in the background and the users never even noticed. A new hot-spare was fitted in the place of the removed dead drive - all without taking the system down or interruping the users. They client was very happy they'd gone for the hot spare! :-)

MotoX.

>
>Dominic
>
>> Regards,
>>
>> Steve Phelan (MotoX).
>>
>> > Regards,
>> >
>> > Dominic
>> >
>> > MotoX wrote:
>> >
>> > > That was well timed - I'm just in the middle of benchtesting strip v
>> > > non-strip v single disk v RAID5 for a client. It's a UNIX box, but
the
>> > > outcome *should* be similar on NT. However, *test* you own kit with a
mix of
>> > > the types of transactions *you* are going to perform. Anyway, here
goes
>> > > (35,000,000 row test table):
>> > >
>> > > READING:
>> > >
>> > > One disk, best read rate 10MB/s per disk (one of).
>> > >
>> > > 3 striped disks, 3.4 MB/s per disk. If parallel query used (degree of
2 or
>> > > 4), rate goes to 5.7 MB/s per disk. In other words, if you stripe
(64k),
>> > > it's also best to use parallel query when you can, because if you
don't you
>> > > won't max out the I/O bandwidth you've created.
>> > >
>> > > On 6 striped disks, 1.95 MB/s per disk. With degree of 2 or 4, 4.4
MB/s per
>> > > disk. In other words, still use parallel, and although you continue
to get a
>> > > boost, it's reducing.
>> > >
>> > > RAID5 - 1 processor, 8.3 MB/s per RAID set. Parallel with degree of 2
or 4,
>> > > 16.9 MB/s per RAID set. In other words, for reading, RAID5 is just as
quick
>> > > as striped disks, but you still need to go parallel to max out the
I/O. And,
>> > > of course, RAID5 (like striping) is much faster than non-RAID single
disks,
>> > > as long as you can provide the CPU to drive it.
>> > >
>> > > WRITING (in summary as I haven't finished yet):
>> > >
>> > > 3 striped drives are about twice as fast as a single disk (not three
times
>> > > faster).
>> > > 6 striped drives are about 4.5 times faster than a single disk.
>> > > NB. Like reading, the overhead of sync'ing the drives, memory
bandwidth,
>> > > etc., affects how much real improvement you get - it won't be linear
as you
>> > > add more drives.
>> > >
>> > > RAID5 - on my limited testing so far, writing (creating a 4 Gig
tablespace)
>> > > has dropped from 3 x 6.8 MB/s per disk to 7.5 MB/s per RAID set. In
other
>> > > words, RAID5 array is over twice as slow on heavy writing as the same
disks
>> > > striped (non RAID5).
>> > >
>> > > In summary, look at striped disks (RAID0 or RAID0+1 or through an
LVM) for
>> > > write intensive parts of your db. Look at RAID5 for read intensive
sections,
>> > > and learn to live with the write penalty where you can.
>> > >
>> > > MotoX.
>> > >
>> > > Dominic Baines wrote in message

<35AA0A1F.41151B15_at_hermes.cam.ac.uk>...
>> > > >I've an NT admin who RAID's everything on every NT box he touches.
This
>> > > >is OK for redundancy if the system dies but I'm not sure this is the
>> > > >best option for an Oracle database performance reasons mainly. Are
there
>> > > >any tuning guru's out there who could shine a like on the
appropriate
>> > > >arguament to use IF it would be better to use individual drives
rather
>> > > >than to rely on the RAID set ?
>> > > >
>> > > >What am I talking about ?:
>> > > >
>> > > >Standard set-up would be an Oracle server on NT 4 SP3 with system
disk
>> > > >a mirrored pair and second 'drive' being a raid set over say 7 to
15
>> > > >disks. So ORANT is on D:, as is everything else. OK there are
different
>> > > >drives and spindles but every read or write to anywhere on the RAID
set
>> > > >would use every drive, wouldn't it ? These are 4GB Seagate SCSI
drives.
>> > > >The controller is a proper RAID controller, whatever that may be.
It's
>> > > >all on Compaq kit.
>> > > >
>> > > >Thanks,
>> > > >
>> > > >Dominic Baines
>> > > >
>
>
>
Received on Thu Jul 16 1998 - 03:10:16 CDT