Re: Linux 2.6.29

Ric Wheeler
2009-03-25T18:57:58Z

Alan Cox wrote:
>> Again, good SSD's don't care. Disks do. It doesn't matter if you have a FC
>> disk array that can eat 300MB/s when streaming - once you start seeking,
>> that 300MB/s goes down like a rock. Battery-protected write caches will
>> help - but not a whole lot when streaming more data than they have RAM.
>> Basic queuing theory.
>>
>
> Subtly more complex than that. If your mashed up I/O streams fit into the
> 2GB or so of cache (minus one stream to disk) you win. You also win
> because you take a lot of fragmented OS I/O and turn it into bigger
> chunks of writing better scheduled. The latter win arguably shouldn't
> happen but it does occur (I guess in part that says we suck) and it
> occurs big time when you've got multiple accessors to a shared storage
> system (where the host OS's can't help)
>
> Alan
>

The other thing that can impact random writes on arrays is their
internal "track" size - if the random write is of a partial track, it
forces a read-modify-write with a back end disk read. Some arrays have
large internal tracks, others have smaller ones.

Again, not unlike what you see with some SSD's and their erase block
size - give them even multiples of that and they are quite happy.

Ric