Re: type of striping

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Wed, 08 Dec 2004 22:19:57 +1100
Message-ID: <41b6e35c$0$14383$afc38c87@news.optusnet.com.au>

Noons wrote:
> Howard J. Rogers wrote:
>
>

>>Because if you choose your uniform size poorly (ie, too small), then

>
> the
>

>>number of extents a segment acquires will shoot up. And the ASSM

>
> bitmaps
>

>>  get more burdensome the more extents they have to manage/deal with.

>
>
> Yes, but that would be the case as well with LMT without ASSM,
> wouldn't it? I mean, it uses a bitmap as well to allocate
> space in uniform extents.

No. You're confusing (I think) the 64K bitmap which handles extent allocations for the tablespace as a whole with the per-extent (or thereabouts) ASSM bitmap blocks. If you have less than about 300,000 extents in the tablespace, then the original 64K LMT bitmap will still do you fine service. But the more extents a particular segment has in ASSM tablespace, the more ASSM bitmap blocks there are (it isn't one per extent, but it sure appears to start off that way if you're not careful).

So no, an LMT without ASSM, if you let the number of extents rip to high heaven, will still have one 64K bitmap allocating the lot.

But an LMT with ASSM, will have one 64K bitmap AND a huge number of ASSM BMB's (Bitmap blocks).

Hence, autoallocate can help: as the number of extents shoots skywards, it starts increasing the extent sizes, so a segment begins to acquire fewer and fewer large extents. Therefore, the same 64K tablespace bitmap, but not so many ASSM BMBs.

> Too small an extent, too large a bitmap,
> *potential* problem. Does it get compounded with ASSM?

For the LMT bitmap, ASSM or no ASSM, to be inadequate, you have to have in excess of about 300,000 extents. It isn't likely to happen any time soon. But as the number of extents for a particular and individual segment approaches the 2,000 mark, the number of BMBs just gets silly.

Two different bitmaps. Two different issues. ASSM makes worrying about the number of extents an issue again. Autoallocate reduces the number of extents a segment acquires. Therefore ASSM and autoallocate are natural bedfellows.

Regards
HJR Received on Wed Dec 08 2004 - 05:19:57 CST