Re: a fast bloat measurement tool (was Re: Measuring relation free space)

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.

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)? Also, autovacuum is not the only place
where we free space - we'd have to handle HOT for example, I guess.

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