Hi!
>>>>> "Scott" == Scott Hess <scott@stripped> writes:
Scott> On Thu, Mar 02, 2000 at 07:27:30PM +0200, Michael Widenius wrote:
>> >>>>> "Peter" == Peter Zaitsev <pz@stripped> writes:
Peter> Other important problem is DATA fragmentation. Well this
Peter> should be the mostly FS problem but I do not think it is
Peter> solved much in EXT2 for example. I have about 20.000 of
Peter> which most are continualy grothin simultaneosely. In
Peter> this case standart space allocation mechanisms will produce
Peter> a hard drive fragmentation if I'm not mistaken.
>>
Peter> The nice thing would be allowing to specify with create
Peter> table something like "groth chunk" which may decrease
Peter> fragmentation.
>>
>> The problem with the above is mainly that this would confuse
>> myisamchk a lot.
Scott> Why? Couldn't it just be allocated as empty space, as if you added
Scott> a record and then deleted it? Even if myisamchk truncated it away,
Scott> that's no big deal (because you don't really run myisamchk all that
Scott> often).
If I remember correctly. the main problem was that when we preallocated
a lot of files, isamchk had to do a lot of extra work to check
the 'empty' space for possible rows; Nothing really serious but at
that time it was more than it was worth...
>> (The MyISAM table format has actually reserved space for relocation
>> information so it wouldn't be that hard to fix this..)
Scott> The similar thing I would like to see is a parameter which indicates how
Scott> much space you're willing to waste to prevent fragmentation. If the
Scott> percent of wasted space were lower than the parameter and there was
Scott> no single piece of contiguous space where mysql could write a record,
Scott> it would then just write the record to the end of the file rather than
Scott> breaking it across the available space.
MyISAM doesn't hold pointer to blocks of free blocks of different
size; It has keeps a link to a free block list. To do the above we
would need to add a lot of pointers (16?) to point to blocks of
different sizes; Not that hard, but it would break the current MyISAM
format :(
Scott> On a small/personal system, you might set this to 0% to invoke the current
Scott> space-saving behaviour. On a beefy RAID array with 3-4x as much space
Scott> as you need for the actual data, you might set this to 100%, wasting
Scott> space in the interests of reducing fragmentation.
Scott> [This is the place we're in on one production system. We needed to
Scott> RAID 6 fast drives to get the seek performance we needed, but we
Scott> really didn't need 100Gig of disk space. It's just hard to buy
Scott> smaller drives these days...]
Regards,
Monty
