On Wed, 2 Mar 2005, Muthian Sivathanu wrote:
Hi,
I have a question on ext3 journal commit code. When a transaction is committed in the ordered mode, ext3 first issues the data writes, waits for them to finish, then issues the journal writes, waits for them to finish, and then writes out the commit record.
It appears that the first wait (for the data blocks) is unnecessary because all that is required is that
Wrong. If you perform two buffered writes back-to-back will you guarantee that they are both on the disk when the second finishes? Not on your life! They can (read will) be reordered depending upon the closest seek. So it is mandatory that one wait to make sure that both writes occur in order.
before the commit, both the data and the metadata blocks should be on disk. This extra wait can potentially reduce performance in cases where the journal is on a separate disk, because you lose parallelism between data writes and the metadata writes.
Does anyone have an idea as to why this extra wait was introduced?
thanks very much, Muthian
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