Below: brilliant article on why NetApp designed its cache product to only affect reads.
http://communities.netapp.com/community/netapp-blogs/efficiency/blog/2011/02/08/flash-cache-doesnt-cache-writes--why
Update: Since writes in ONTAP require several reads to accomplish (updating bitmaps, etc), flashcache can speed up writes. But since that data is often in system memory already, this is complicated and depends on the situation.
http://communities.netapp.com/community/netapp-blogs/efficiency/blog/2011/02/08/flash-cache-doesnt-cache-writes--why
Update: Since writes in ONTAP require several reads to accomplish (updating bitmaps, etc), flashcache can speed up writes. But since that data is often in system memory already, this is complicated and depends on the situation.
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