Revert "increased readahead buffer to 1024 (had reports of sluggish I/O, and bruce's kernel had it set to default (128) )"

This reverts commit 6fd6c512a834990fe33350471d4147d4b3968fe9.

Change-Id: I8a91d2bce35f4c8f4d5b91ec6318c68c90f0f277