From: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Dilip Kumar <dilipbalaut(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, David Steele <david(at)pgmasters(dot)net>, Ildus Kurbangaliev <i(dot)kurbangaliev(at)gmail(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: [HACKERS] Custom compression methods (mac+lz4.h) |
Date: | 2021-03-21 23:43:24 |
Message-ID: | 20210321234324.GC4203@telsasoft.com |
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Lists: | pgsql-hackers |
On Sun, Mar 21, 2021 at 07:11:50PM -0400, Tom Lane wrote:
> I wrote:
> > Justin Pryzby <pryzby(at)telsasoft(dot)com> writes:
> >> On Sun, Mar 21, 2021 at 04:32:31PM -0400, Tom Lane wrote:
> >>> This seems somewhat repeatable (three identical failures in three
> >>> attempts). Not sure why I did not see it yesterday; but anyway,
> >>> there is something wrong with partial detoasting for LZ4.
>
> >> With what version of LZ4 ?
>
> > RHEL8's, which is
> > lz4-1.8.3-2.el8.x86_64
>
> I hate to be the bearer of bad news, but this suggests that
> LZ4_decompress_safe_partial is seriously broken in 1.9.2
> as well:
>
> https://github.com/lz4/lz4/issues/783
Ouch
> Maybe we cannot rely on that function for a few more years yet.
>
> Also, I don't really understand why this code:
>
> /* slice decompression not supported prior to 1.8.3 */
> if (LZ4_versionNumber() < 10803)
> return lz4_decompress_datum(value);
>
> It seems likely to me that we'd get a flat out build failure
> from library versions lacking LZ4_decompress_safe_partial,
> and thus that this run-time test is dead code and we should
> better be using a configure probe if we intend to allow old
> liblz4 versions. Though that might be moot.
The function existed before 1.8.3, but didn't handle slicing.
https://github.com/lz4/lz4/releases/tag/v1.8.3
|Finally, an existing function, LZ4_decompress_safe_partial(), has been enhanced to make it possible to decompress only the beginning of an LZ4 block, up to a specified number of bytes. Partial decoding can be useful to save CPU time and memory, when the objective is to extract a limited portion from a larger block.
Possibly we could allow v >= 1.9.3 || (ver >= 1.8.3 && ver < 1.9.2).
Or maybe not: the second half apparently worked "by accident", and we shouldn't
need to have intimate knowledge of someone else's patchlevel releases,
--
Justin
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