Re: a fast bloat measurement tool (was Re: Measuring relation free space) - Mailing list pgsql-hackers

On 24.2.2015 19:08, Jim Nasby wrote:
> On 2/22/15 8:32 PM, Tomas Vondra wrote:
>> On 23.2.2015 03:20, Jim Nasby wrote:
>>> On 2/22/15 5:41 PM, Tomas Vondra wrote:
>>>> Otherwise, the code looks OK to me. Now, there are a few features I'd
>>>> like to have for production use (to minimize the impact):
>>>>
>>>> 1) no index support:-(
>>>>
>>>>      I'd like to see support for more relation types (at least btree
>>>>      indexes). Are there any plans for that? Do we have an idea on
>>>> how to
>>>>      compute that?
>>>
>>> It'd be cleaner if had actual an actual am function for this, but see
>>> below.
>>>
>>>> 2) sampling just a portion of the table
>>>>
>>>>      For example, being able to sample just 5% of blocks, making it
>>>> less
>>>>      obtrusive, especially on huge tables. Interestingly, there's a
>>>>      TABLESAMPLE patch in this CF, so maybe it's possible to reuse some
>>>>      of the methods (e.g. functions behind SYSTEM sampling)?
>>>>
>>>> 3) throttling
>>>>
>>>>      Another feature minimizing impact of running this on production
>>>> might
>>>>      be some sort of throttling, e.g. saying 'limit the scan to 4 MB/s'
>>>>      or something along those lines.
>>>>
>>>> 4) prefetch
>>>>
>>>>      fbstat_heap is using visibility map to skip fully-visible pages,
>>>>      which is nice, but if we skip too many pages it breaks readahead
>>>>      similarly to bitmap heap scan. I believe this is another place
>>>> where
>>>>      effective_io_concurrency (i.e. prefetch) would be appropriate.
>>>
>>> All of those wishes are solved in one way or another by vacuum and/or
>>> analyze. If we had a hook in the tuple scanning loop and at the end of
>>> vacuum you could just piggy-back on it. But really all we'd need for
>>> vacuum to be able to report this info is one more field in LVRelStats, a
>>> call to GetRecordedFreeSpace for all-visible pages, and some logic to
>>> deal with pages skipped because we couldn't get the vacuum lock.
>>>
>>> Should we just add this to vacuum instead?
>>
>> Possibly. I think the ultimate goal is to be able to get this info
>> easily and without disrupting the system performance too much (which is
>> difficult without sampling/throttling). If we can stuff that into
>> autovacuum reasonably, and then get the info from catalogs, I'm OK with
>> that.
> 
> Doing the counting in vacuum/analyze (auto or not) is quite easy, and it
> would happen at the same time we're doing useful work. We would
> automatically get the benefit of the throttling and sampling work that
> those routines already do.
> 
>> However I'm not sure putting this into autovacuum is actually possible,
>> because how do you merge data from multiple partial runs (when each of
>> them skipped different pages)?
> 
> ISTM that's just a form of sampling, no?

Maybe.

I was thinking about collecting the necessary info during the VACUUM
phase, and somehow keeping track of free space in the whole table. I
thought there would be trouble exactly because this phase only processes
pages that possibly need vacuuming (so it wouldn't be a truly random
sample, making the estimation tricky).

But maybe that's not really true and it is possible to do that somehow.
For example what if we kept track of how much space each VACUUM freed,
and keeping running sum?

It might also be done during the ANALYZE, but that seems a bit
complicated because that's based on a sample of rows, not pages. Also,
the autovacuum_analyze_factor is 0.2 by default, so could end up with up
to 20% bloat without knowing it (vacuum_factor=0.1 is not great either,
but it's better).


> Besides, we don't need the same lock for figuring out bloat. We
> could still measure bloat even if we can't vacuum the page, but I
> think that's overkill. If we're skipping enough pages to mess with
> the bloat measurement then we most likely need to teach vacuum how to
> revisit pages.
> 
>> Also, autovacuum is not the only place
>> where we free space - we'd have to handle HOT for example, I guess.
> 
> I wasn't thinking about trying to keep live bloat statistics, so HOT
> wouldn't affect this.

I'm afraid this might cause the estimate to drift away over time, but I
guess it depends on implementation - e.g. if doing this in ANALYZE, it'd
be mostly immune to this, while with collecting incremental info during
VACUUM it might be a problem I guess.


Anyway, we don't have a patch trying to do that (certainly not in this
CF). I think it makes sense to add fastbloat() to pageinspect. Maybe
we'll get autovacuum-based solution in the future, but we don't have
that right now.

Actually, wouldn't that be a nice GSoC project? The scope seems about
right, not touching too many parts of the code base, etc.


-- 
Tomas Vondra                http://www.2ndQuadrant.com/
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