Re: (ITS#8156) triggering MDB_PAGE_FULL
by sergej.jurecko＠gmail.com
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Thank you for the explanation.
Sergej
On Wed, May 27, 2015 at 10:32 AM, Hallvard Breien Furuseth <
h.b.furuseth(a)usit.uio.no> wrote:
> On 27/05/15 09:00, sergej.jurecko(a)gmail.com wrote:
>
>> Using a dupsort and writing a large value (which is still bellow
>> maxkeysize)
>> will result in MDB_PAGE_FULL.
>> (...)
>> maxkeysize=10000
>>
>
> Too big MDB_MAXKEYSIZE. Its doc says:
> "Keys and MDB_DUPSORT data items must fit on a node in a regular page."
>
> But the doc should clarify that MDB_MAXKEYSIZE can be set big enough
> to break lmdb.
>
> Or mdb_env_open() can fail if the computed me_maxkey would be smaller.
>
> Or compute a safe MIN_PAGESIZE from MDB_MAXKEYSIZE and use that,
> and then fail if the page size would be unsupported. (MIN_PAGESIZE
> can be computed at compile time, with some ugly macros.)
>
> lmdb sets the max safe maxkeysize if you define MDB_MAXKEYSIZE=0.
> Some time ago, that was 1/3 of the size you could reasonably use
> (to allow 1 key + sub-page with 2 dupsort values), so it could make
> sense to define MDB_MAXKEYSIZE above the safe max. Today lmdb
> uses a sub-DB at once if a sub-page would be too small, though.
> So I don't see much need to support maxkeysize > safe max.
>
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<div dir=3D"ltr"><div>Thank you for the explanation. <br><br><br></div>Serg=
ej<br></div><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On We=
d, May 27, 2015 at 10:32 AM, Hallvard Breien Furuseth <span dir=3D"ltr"><=
;<a href=3D"mailto:h.b.furuseth@usit.uio.no" target=3D"_blank">h.b.furuseth=
@usit.uio.no</a>></span> wrote:<br><blockquote class=3D"gmail_quote" sty=
le=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">On 27/=
05/15 09:00, <a href=3D"mailto:sergej.jurecko@gmail.com" target=3D"_blank">=
sergej.jurecko(a)gmail.com</a> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">
Using a dupsort and writing a large value (which is still bellow=C2=A0 maxk=
eysize)<br>
will result in MDB_PAGE_FULL.<br>
(...)<br>
maxkeysize=3D10000<br>
</blockquote>
<br>
Too big MDB_MAXKEYSIZE.=C2=A0 Its doc says:<br>
"Keys and MDB_DUPSORT data items must fit on a node in a regular page.=
"<br>
<br>
But the doc should clarify that MDB_MAXKEYSIZE can be set big enough<br>
to break lmdb.<br>
<br>
Or mdb_env_open() can fail if the computed me_maxkey would be smaller.<br>
<br>
Or compute a safe MIN_PAGESIZE from MDB_MAXKEYSIZE and use that,<br>
and then fail if the page size would be unsupported.=C2=A0 (MIN_PAGESIZE<br=
>
can be computed at compile time, with some ugly macros.)<br>
<br>
lmdb sets the max safe maxkeysize if you define MDB_MAXKEYSIZE=3D0.<br>
Some time ago, that was 1/3 of the size you could reasonably use<br>
(to allow 1 key + sub-page with 2 dupsort values), so it could make<br>
sense to define MDB_MAXKEYSIZE above the safe max.=C2=A0 Today lmdb<br>
uses a sub-DB at once if a sub-page would be too small, though.<br>
So I don't see much need to support maxkeysize > safe max.<br>
</blockquote></div><br></div>
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