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IDS Forum

Re: Outrageous Extent sizes

Posted By: david@smooth1.co.uk
Date: Thursday, 24 August 2017, at 2:00 p.m.

In Response To: Re: Outrageous Extent sizes (MICHAEL HOFFMAN)

The general idea is to monitor this and not let the number of extents get out of hand.

Plan space usage and increase the next extent size as needed.

Also use alter fragment to rebuild the table with larger extents then if it is known the table will not grow much then reduce the next extent size.

Regards,
David.

> On 24 August 2017 at 18:48 MICHAEL HOFFMAN <offdisc@gmail.com> wrote:
>
>
> Evidence of runaway extents for Table C:
> >From sysmaster.sysptext table, you see the sizes double at regular intervals
> (nearly every 16th, as expected). Then the extent sizes are all over the place
> due to lack of contiguous free space to hold a larger extent. Understandable!
> But then the final extent allocated is so incredibly large compared to any
> others (80MB --> 640MB!).
>
> But now I see .... even though the document says the doubling happens at
> *every allocation*, it still seems to have an "every 16th" pattern. At 128
> extents, the size has doubled to 640MB. So at extent 160, the doubling could
> take it to 2.5GB, but since 640MB would provide more than 10% free of the
> overall table size, Informix stops there.
>
> This aggressive approach can be a real headache for DBAs running on systems
> that are space-constrained. A large table that rarely grabs a new extent can
> all of a sudden snag a HUGE amount of space, even if the table is rarely
> written to.
>
> And does the NEXT EXTENT have *any* meaning at all anymore?
>
> pe_extnum pe_chunk pe_size
> ---------- -------- --------
>
> 0 30 8
>
> 1 48 796160
>
> 2 30 1280
>
> 3 30 1280
>
> 4 30 1280
>
> 5 30 1280
>
> 6 30 1280
>
> 7 30 1280
>
> 8 30 1280
>
> 9 30 1280
>
> 10 30 1280
>
> 11 30 1280
>
> 12 30 1280
>
> 13 30 1280
>
> 14 30 1280
>
> 15 30 1280
>
> 16 30 2560
>
> 17 30 2560
>
> 18 30 2560
>
> 19 30 2560
>
> 20 30 2560
>
> 21 30 2560
>
> 22 30 2560
>
> 23 30 2560
>
> 24 30 2560
>
> 25 30 2560
>
> 26 30 2560
>
> 27 30 2560
>
> 28 30 5120
>
> 29 30 2560
>
> 30 30 5120
>
> 31 30 2560
>
> 32 30 5120
>
> 33 38 5120
>
> 34 38 5120
>
> 35 38 5120
>
> 36 38 5120
>
> 37 38 5120
>
> 38 38 5120
>
> 39 38 5120
>
> 40 38 5120
>
> 41 38 5120
>
> 42 38 5120
>
> 43 38 5120
>
> 44 38 5120
>
> 45 38 5120
>
> 46 38 5120
>
> 47 38 5120
>
> 48 38 10240
>
> 49 38 10240
>
> 50 38 10240
>
> 51 38 10240
>
> 52 48 10240
>
> 53 48 10240
>
> 54 48 10240
>
> 55 48 10240
>
> 56 48 10240
>
> 57 48 10240
>
> 58 48 10240
>
> 59 48 10240
>
> 60 48 10240
>
> 61 48 10240
>
> 62 48 10240
>
> 63 48 10240
>
> 64 48 20480
>
> 65 48 20480
>
> 66 55 20480
>
> 67 55 20480
>
> 68 55 20480
>
> 69 55 20480
>
> 70 55 20480
>
> 71 55 20480
>
> 72 55 20480
>
> 73 55 20480
>
> 74 55 20480
>
> 75 55 20480
>
> 76 30 20480
>
> 77 30 20480
>
> 78 30 20480
>
> 79 30 20480
>
> 80 38 40960
>
> 81 48 40960
>
> 82 48 40960
>
> 83 55 40960
>
> 84 55 40960
>
> 85 55 40960
>
> 86 55 40960
>
> 87 55 40960
>
> 88 55 40960
>
> 89 55 40960
>
> 90 55 40960
>
> 91 48 30976
>
> 92 38 19484
>
> 93 48 18048
>
> 94 48 16416
>
> 95 38 16023
>
> 96 55 15182
>
> 97 48 13823
>
> 98 38 13048
>
> 99 38 12288
>
> 100 48 11248
>
> 101 38 10876
>
> 102 48 9846
>
> 103 48 9524
>
> 104 38 8262
>
> 105 48 8040
>
> 106 48 7765
>
> 107 38 6854
>
> 108 48 6391
>
> 109 30 6263
>
> 110 48 6152
>
> 111 38 5760
>
> 112 55 5572
>
> 113 38 5442
>
> 114 38 5146
>
> 115 38 4842
>
> 116 55 4704
>
> 117 55 4589
>
> 118 48 4464
>
> 119 48 4272
>
> 120 30 4096
>
> 121 55 4096
>
> 122 48 3978
>
> 123 48 3936
>
> 124 38 3530
>
> 125 38 3264
>
> 126 48 3121
>
> 127 30 3087
>
> 128 38 3084
>
> 129 38 3072
>
> 130 38 3008
>
> 131 48 2868
>
> 132 55 2680
>
> 133 48 2640
>
> 134 48 2608
>
> 135 38 2495
>
> 136 48 2372
>
> 137 38 2280
>
> 138 48 2235
>
> 139 38 2158
>
> 140 38 2024
>
> 141 30 1870
>
> 142 38 1715
>
> 143 38 1681
>
> 144 30 1628
>
> 145 48 1608
>
> 146 48 1600
>
> 147 48 1540
>
> 148 48 1534
>
> 149 38 1450
>
> 150 48 1419
>
> 151 48 1408
>
> 152 30 1376
>
> 153 48 1328
>
> 154 38 1282
>
> 155 30 1206
>
> 156 30 1144
>
> 157 48 1069
>
> 158 30 1068
>
> 159 38 1068
>
> 160 76 327680
>
> > --------------------------------------------
> > Thanks Art. That helps explain part of the issue -- especially with the
> > larger table. 976.5 MB *2 = 1.90 GB.
> >
> > However, the smaller table still makes no sense. And now I have evidence
> > of another table having the same issue.
> >
> > Table C: 4.3 GB needing another extent. NEXT size is 2.5 MB.
> >
> > According to the document, the NEXT size should be doubled, up to 128KB,
> > and once there, doubled only when "the remaining space in the table is
> > less than 10% of the total allocated space in the table".
> >
> > So the NEXT extent should be 5 MB, right? Well then, why is the new extent
> > **640 MB**? That is 15% of the table size.
> >
> > Is Informix continuing to double the NEXT size until it crosses the 10%
> > threshold before actually allocating the extent?
> >
> > That would actually explain the behavior for Table A as well (1.25GB is 13%
> > of 9.5GB)
> >
> > The documentation is definitely not clear on this. (Not to mention the
> > potentially wasted Free space for tables that may be front-loaded with data,
> > but barely grow in the future.)
> >
> > Thanks,
> > Michael
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>

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