On Sun, Aug 15, 2010 at 2:58 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> In the discussion of bug #5599 we pretty much agreed to do this:
>> Seems like we need to think harder about recovering from a truncate
>> failure. A few random ideas:
>> 1. Write the dirty buffers before dropping them. Kind of ugly from a
>> performance viewpoint, but simple and safe.
>
> I looked at making this happen, and noted that DropRelFileNodeBuffers
> is used both for the truncation case and for dropping relation buffers
> during smgrdounlink. In the latter case, it's still appropriate to
> drop dirty buffers without writing them, both for performance reasons
> and because we don't really care about any errors: we have already
> committed the relation DROP, and are not going to look at the file
> contents again in any case. So this means that two different behaviors
> are now required for truncation and dropping.
>
> The cleanest fix is an API change to add a boolean write-or-not
> parameter to DropRelFileNodeBuffers. That's what I want to do in HEAD
> and 9.0, but I'm unsure whether it's a safe fix in the back branches.
> Does anyone have an opinion whether it's likely that any third-party
> code is calling DropRelFileNodeBuffers directly? If there is, then
> changing its API in a minor release would be an unfriendly thing to do.
> We could avoid that by some ugly expedient like inserting a second copy
> of the function in back branches.
>
> Comments?
I really hate this solution, because writing out data that we're about
to throw away just in case we can't actually throw it away seems like
a real waste from a performance standpoint. Could we avoid this
altogether by allocating a new relfilenode on truncate?
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
