[Jdbm-developer] Re: VLR Tx, MVCC and DBCache

[Alex B. <boi...@in...>]

Thompson, Bryan B. wrote:

>Let's assume that jdbm uses locking for rw synchronization and MVCC for
>ww synchronization.
>  
>
I don't understand what you mean here.

>This means that updated rows will NOT be updated in
>place.  Instead a new version of the row will be created by logically
>appending the row to the store (either reusing a deleted physical row or
>using an empty page to lay down new physical rows).
>  
>
Ok

>In the case of a VLR Tx, isolation of the new rows on unused pages will
>ensure that the DBCache policy of exclusive access to the VLR Tx pages
>does not conflict with the access of other transactions to pre-existing
>versions.
>  
>
You also need isolation of row mapping pages.

>This further suggests to me that we should buffer serialized rows so
>that (a) we can cluster them and (b) we can decide whether to use some
>rows from the deleted physical rows list or begin allocation on an unused
>page.  If the buffer is not filled before the tx commits, then we can use
>some deleted physical rows since we know that the tx will not be promoted
>to VLR status.
>
Yes.  Do keep in mind that you can only reuse physical rows if no
transaction can ever reach them.  This requires some additional
housekeeping.

>If the buffer is filled then the tx may be promoted to
>VLR status, so we would allocate on unused pages (one page at a time) to
>ensure that a conflict does not arise with the DBCache policy of exclusive
>access for a VLR Tx to pages which it logs and then forces to the DB.
>  
>
When and how would you update the mapping pages?

alex