Re: Block at a time ...

This is why pre-allocation is a good idea if you have the space ....

Tom, what about a really simple command in a forthcoming release of PG that
would just preformat a 1GB file at a time? This is what I've always done
scripted with Oracle (ALTER TABLESPACE foo ADD DATAFILE ....) rather than
relying on its autoextender when performance has been a concern.

Cheers
Dave

On Mon, Mar 22, 2010 at 3:55 PM, Pierre C <lists(at)peufeu(dot)com> wrote:

>
> This is one of the more-or-less solved problems in Unix/Linux. Ext* file
>>> systems have a "reserve" usually of 10% of the disk space that nobody except
>>> root can use. It's not for root, it's because with 10% of the disk free,
>>> you can almost always do a decent job of allocating contiguous blocks and
>>> get good performance. Unless Postgres has some weird problem that Linux has
>>> never seen before (and that wouldn't be unprecedented...), there's probably
>>> no need to fool with file-allocation strategies.
>>>
>>> Craig
>>>
>>
>> Its fairly easy to break. Just do a parallel import with say, 16
>> concurrent tables being written to at once. Result? Fragmented tables.
>>
>
> Delayed allocation (ext4, XFS) helps a lot for concurrent writing at a
> medium-high rate (a few megabytes per second and up) when lots of data can
> sit in the cache and be flushed/allocated as big contiguous chunks. I'm
> pretty sure ext4/XFS would pass your parallel import test.
>
> However if you have files like tables (and indexes) or logs that grow
> slowly over time (something like a few megabytes per hour or less), after a
> few days/weeks/months, horrible fragmentation is an almost guaranteed result
> on many filesystems (NTFS being perhaps the absolute worst).
>
>
>
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