On Sun, Jun 15, 2014 at 2:51 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: > On Thu, Jun 12, 2014 at 9:31 AM, Gurjeet Singh <gurj...@singh.im> wrote: >> >> I don't have intimate knowledge of recovery but I think the above >> assessment of recovery's operations holds true. If you still think >> this is a concern, can you please provide a bit firm example using >> which I can visualize the problem you're talking about. > > Okay, let me try with an example: > > Assume No. of shared buffers = 5 > Before Crash: > > 1. Pages in shared buffers numbered 3, 4, 5 have write operations > performed on them, followed by lot of reads which makes their > usage_count as 5. > 2. Write operation happened on pages in shared buffers numbered > 1, 2. Usage_count of these buffers is 1. > 3. Now one read operation needs some different pages and evict > pages in shared buffers numbered 1 and 2 and read the required > pages, so buffers 1 and 2 will have usage count as 1. > 4. At this moment shutdown initiated. > 5. Bgwriter saved just buffers 1 and 2 and crashed. > > After Crash: > > 1. Recovery will read in pages on which operations happened in > step-1 and 2 above. > 2. Buffer loader (pg_hibernator) will load buffers on which operations > happened in step-3, so here it might needs to evict buffers which are > corresponding to buffers of step-1 before crash. So what this > essentially means is that pg_hibernator can lead to eviction of more > useful pages.
Granted, you have demonstrated that the blocks restored by pg_hibernator can cause eviction of loaded-by-recovery blocks. But, one can argue that pg_hibernator brought the shared-buffer contents to to a state that is much closer to the pre-shutdown state than the recovery would have restored them to. I think this supports the case for pg_hibernator, that is, it is doing what it is supposed to do: restore shared-buffers to pre-shutdown state. I agree that there's uncertainty as to which buffers will be cleared, and hence which blocks will be evicted. So pg_hibernator may cause eviction of blocks that had higher usage count before the shutdown, because they may have a lower/same usage count as other blocks' buffers after recovery. There's not much that can be done for that, because usage count information is not saved anywhere on disk, and I don't think it's worth saving just for pg_hibernator's sake. Best regards, -- Gurjeet Singh http://gurjeet.singh.im/ EDB : www.EnterpriseDB.com : The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
