Re: [Exadata] How do you use FlashDisk ?

From: Nuno Souto <dbvision_at_iinet.net.au>
Date: Tue, 22 Mar 2011 22:20:26 +1100
Message-ID: <4D8885FA.3080606_at_iinet.net.au>



No, don't do some maths. Without sufficient information, it's a wasted exercise.

"So I do not see where the EMC guys are wrong unless they know we are talking 1 to 100 ratio so increasing the cache even by 4-5 times will be a drop in the bucket."

pretty much sums it up. The problem of course is that the EMC guys started by claiming the cache can do wonders for everything, including bend bananas. Not once did they seek to find out the real data dimensions we were talking about. As in: find out what the data is *before* using it? And of course they conveniently "abstracted" the cache is not totally dedicated to a single I/O from a single process when the SAN is shared by around 25 servers...

-- 
Cheers
Nuno Souto
in sunny Sydney, Australia
dbvision_at_iinet.net.au

Yechiel Adar wrote,on my timestamp of 21/03/2011 11:44 PM:

> Lets do some math.
>
> You have 10 MB of write back cache and you need to write 50 MB of data.
> Assuming 1 ms for write to the cache and 10 ms to write to the disk, you need
> and each i/o is 8 KB:
>
> You fill the cache with 112 i/o in 112 ms and save about 1000 ms from the write
> time.
> Instead of waiting 5500 ms you wait only 4500ms, 20% saving.
> If you have double the cache you save 40%.
>
> It all depends on the ratio of your total writes vs the cache size.
>
> So I do not see where the EMC guys are wrong unless they know we are talking 1
> to 100 ratio so increasing the cache even by 4-5 times will be a drop in the
> bucket.
>
> Yechiel Adar
> Israel
>
>
> On 20/03/2011 12:48, Nuno Souto wrote:
>> Off course, when doing large sequentials writes that write cache will not help
>> you much.
>>
>> Can someone please let the "experts" at EMC know this?
> --
-- http://www.freelists.org/webpage/oracle-l
Received on Tue Mar 22 2011 - 06:20:26 CDT

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