Re: Streaming I/O, vectored I/O (WIP)

On Sat, Mar 9, 2024 at 3:55 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
Hi Thomas,

I am planning to review this patch set, so started going through 0001,
I have a question related to how we are issuing smgrprefetch in
StartReadBuffers()

+ if (operation->io_buffers_len > 0)
+ {
+ if (flags & READ_BUFFERS_ISSUE_ADVICE)
{
- if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
- {
- ereport(WARNING,
- (errcode(ERRCODE_DATA_CORRUPTED),
- errmsg("invalid page in block %u of relation %s; zeroing out page",
- blockNum,
- relpath(smgr->smgr_rlocator, forkNum))));
- MemSet((char *) bufBlock, 0, BLCKSZ);
- }
- else
- ereport(ERROR,
- (errcode(ERRCODE_DATA_CORRUPTED),
- errmsg("invalid page in block %u of relation %s",
- blockNum,
- relpath(smgr->smgr_rlocator, forkNum))));
+ /*
+ * In theory we should only do this if PrepareReadBuffers() had to
+ * allocate new buffers above. That way, if two calls to
+ * StartReadBuffers() were made for the same blocks before
+ * WaitReadBuffers(), only the first would issue the advice.
+ * That'd be a better simulation of true asynchronous I/O, which
+ * would only start the I/O once, but isn't done here for
+ * simplicity. Note also that the following call might actually
+ * issue two advice calls if we cross a segment boundary; in a
+ * true asynchronous version we might choose to process only one
+ * real I/O at a time in that case.
+ */
+ smgrprefetch(bmr.smgr, forkNum, blockNum, operation->io_buffers_len);
}

This is always issuing smgrprefetch starting with the input blockNum,
shouldn't we pass the first blockNum which we did not find in the
Buffer pool? So basically in the loop above this call where we are
doing PrepareReadBuffer() we should track the first blockNum for which
the found is not true and pass that blockNum into the smgrprefetch()
as a first block right?

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com