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Re: WAL-based allocation of XIDs is insecure

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Ian Lance Taylor <ian(at)airs(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: WAL-based allocation of XIDs is insecure
Date: 2001-03-05 20:15:40
Message-ID: 20503.983823340@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Ian Lance Taylor <ian(at)airs(dot)com> writes:
> I described myself unclearly.  I was suggesting an addition to what
> you are suggesting.  The worst case can not be worse.

Then I didn't (and still don't) understand your suggestion.  Want to
try again?

> If you are going to allocate a few thousand XIDs each time, then I
> agree that my suggested addition is not worth it.  But how do you deal
> with XID wraparound on an unstable system?

About the same as we do now: not very well.  But if your system is that
unstable, XID wrap is the least of your worries, I think.

Up through 7.0, Postgres allocated XIDs a thousand at a time, and not
only did the not-yet-used XIDs get lost in a crash, they'd get lost in
a normal shutdown too.  What I propose will waste XIDs in a crash but
not in a normal shutdown, so it's still an improvement over prior
versions as far as XID consumption goes.

			regards, tom lane

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