Horton

Re: logtape.c stats don't account for unused "prefetched" block numbers

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Jeff Davis <pgsql@j-davis.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-09-14T22:23:59Z
Lists: pgsql-hackers 
On 2020-Sep-14, Peter Geoghegan wrote:

> On Fri, Sep 11, 2020 at 6:37 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > It would be awkward if we just used nBlocksWritten within
> > LogicalTapeSetBlocks() in the case where we didn't preallocate (or in
> > all cases). Not entirely sure what to do about that just yet.
> 
> I guess that that's the logical thing to do, as in the attached patch.

I don't understand this patch.  Or maybe I should say I don't understand
the code you're patching.  Why isn't the correct answer *always*
nBlocksWritten?  The comment in LogicalTapeSet says:

"nBlocksWritten is the size of the underlying file, in BLCKSZ blocks."

so if LogicalTapeSetBlocks wants to do what its comment says, that is,

"Obtain total disk space currently used by a LogicalTapeSet, in blocks."

then it seems like they're an exact match.  Either that, or more than
zero of those comments are lying.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Commits

  1. Change LogicalTapeSetBlocks() to use nBlocksWritten.

  2. HashAgg: release write buffers sooner by rewinding tape.

  3. logtape.c: do not preallocate for tapes when sorting

  4. Fix bogus MaxAllocSize check in logtape.c.

  5. Avoid fragmentation of logical tapes when writing concurrently.

  6. Logical Tape Set: use min heap for freelist.

  7. Don't create "holes" in BufFiles, in the new logtape code.