Re: [Exadata] How do you use FlashDisk ?

  • From: Yechiel Adar <adar666@xxxxxxxxxxxx>
  • Date: Mon, 21 Mar 2011 14:44:44 +0200

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

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?

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