Re: Do extents impact speed?
- From: "Neil Truby" <neil.truby@xxxxxxxxxxx>
- Date: Mon, 5 Sep 2005 23:59:59 +0100
My point was that - and I'm just making surmises here based on what very
little I know about your business - is that your users are unlikely to be
using SAN technology, particualrly with cacheing, and I'd like to know what
effect such environments have on Chris's advice.
Our expereince of upgrading customers' hardware - particualrly on IBM where
I'm most impressed with the Power5 processors - is that Informix performance
is dramatically improved in every case.
"malcolm weallans" <malcolm.iiug@xxxxxxxxxxxxxxx> wrote in message
news:1125925956.1745e8345764e3f90733abbf8a2a1c37@xxxxxxxxxxx
>
> It is exactly because of the advances in technology,memory and disk size,
> that I first asked the question. We are using fully mirrored and striped
> logical volumes on AIX. We only have one or two mirrored disk pairs so
> there seems to be little benefit in putting different
> dbspaces/tables/fragments on different disks. (The only constraint we
> have
> on chunks is a 2Gbyte limit which comes from IDS 9.3 days)
>
> Doing a dbexport/dbimport to defragment is a non-starter. A VAR
> application
> is involved and they do not support the use of dbexport/dbimport. Instead
> they provide their own utility which is much slower but uses much less
> disk
> space.
>
> Also, there are thousands of tables, most of which are unused, but the
> tables that are used vary from site to site. Current performance is
> "acceptable" although nearly all of the customers have commented that the
> new hardware, new OS, IDS instead of SE, has given them no improvement in
> performance - and some say that it is worse.
>
> The only REAL reason I can see for defragmenting the tables is that on a
> few
> tables we are rapidly approaching the maximum number of extents but only
> on
> our Linux platforms.
>
> Regards
>
> Malcolm
>
>
>
> -----Original Message-----
> From: owner-informix-list@xxxxxxxx [mailto:owner-informix-list@xxxxxxxx]
> On
> Behalf Of Neil Truby
> Sent: 03 September 2005 12:43
> To: informix-list@xxxxxxxx
> Subject: Re: Do extents impact speed?
>
>
> "Christopher Coleman" <Christopher.Coleman@xxxxxxxxxxxx> wrote in message
> news:1125707836.27d72252da0b7b38721d279905f4197d@xxxxxxxxxxx
>>
>> Yes, this many extents will almost certainly impact speed for the
>> worst.
>> At the very simple level, there would be a significant increase in disk
>> seek time.
>>
>> Even the Performance Guide says: "Performance suffers when disk seeks
>> for
>> a table must span more than one extent, particularly for sequential
>> scans."
>> http://publib.boulder.ibm.com/epubs/html/25122960/perf189.htm#idx2131
>>
>> An even worse scenario is when the extents are no longer ordered
>> sequentially (i.e.. extent x, extent z, extent y). This means that, in
>> order to read through a table or index, the disk head must move backward
>> and forward, increasing the lag time. This can happen if some tables are
>> dropped or reorganized while other tables continue to grow, a common
>> cause
>
>> of interleaving.
>
> Isn't this making some fairly sweeping assummptions about the nature of
> the
> storage technology in use (I know you went on to make this point in part,
> but I'm playing Devil's Advocate as I'd like to canvass views on how much
> if
>
> at all the above applies to cached SANs). Many users these days -
> although
> possibly not Malcolm's - will be using sophisticated, cached SANs where
> there will be a high level of indeirection between the apparent and actual
> physical layout.
> sending to informix-list
.
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