Re: Silly question, defrag

[Hans Reiser <reiser@namesys.com>]

Benjamin Scott wrote:

>On Fri, 5 Apr 2002, at 12:31am, Ross Vandegrift wrote:
>
>>>I think it would seem likely that, once a filesystem does become fragmented,
>>>performance is going to suck mud.
>>>
>>I'm curious though - shouldn't the elevator read algorithms take care of
>>this, for the most part?  If you wait to do a read until you have a few,
>>and read sequentially across the disk, you won't be hit with any seeks.
>>
>
>  I am no expert, but I suspect there is only so much the elevator can do.  
>For a moderately small read, I image your scenario would work.  But given a
>larger read and enough fragmentation, you are going to have to seek all over
>the place.  Sure, you could increase the wait time in the elevator, and use
>tons of buffers to re-order reads, but you will eventually start spending so
>many resources on the elevator that you are back to a performance problem
>again.
>
Actually, the larger the read, the better the elevator can compensate. 
 The problem is that if you do a whole bunch of separate reads of a 
whole bunch of files, and you wait for each read to complete before 
doing the next one, the elevator cannot do a good job.  Also, if you 
have data you don't want to read interleaved with what you do want to 
read, then your transfer rate will be decreased.

Hans