Re: [HACKERS] Proposal : For Auto-Prewarm.

Few comments on the latest patch:
+
+ /* Prewarm buffer. */
+ buf = ReadBufferExtended(rel, blk->forknum, blk->blocknum, RBM_NORMAL,
+ NULL);
+ if (BufferIsValid(buf))
+ ReleaseBuffer(buf);
+
+ old_blk = blk;
+ ++pos;

You are incrementing position at different places in this loop. I
think you can do it once at the start of the loop.

2.
+dump_now(bool is_bgworker)
{
..
+ fd = OpenTransientFile(transient_dump_file_path,
+   O_CREAT | O_WRONLY | O_TRUNC, 0666);

+prewarm_buffer_pool(void)
{
..
+ file = AllocateFile(AUTOPREWARM_FILE, PG_BINARY_R);

During prewarm, you seem to be using binary mode to open a file
whereas during dump binary flag is not passed.  Is there a reason
for such a difference?

3.
+ ereport(LOG,
+ (errmsg("saved metadata info of %d blocks", num_blocks)));

It doesn't seem like a good idea to log this info at each dump
interval.  How about making this as a DEBUG1 message?