Re: swap storage alignment and stride size

[Ric Wheeler <ricwheeler@gmail.com>]

On 12/08/2010 12:03 PM, Christian Brandt wrote:
> Preamble:
>
> Hi fellow linux tamers, the following question has bounced around for
> some days in local lists and newsgroups without conclussion and was
> escalated upstream several times, here we are...
>
> We are discussing semi-professional storage systems, e.g. ext4 on luks
> on lvm on raid on gpt-partitions on 4k sector harddrives or 512k sector
> SSDs. Usually every level profits a lot from aligning the data to the
> underlying sector/stride/chunk size, e.g. ext4 with a 128k stripe size
> will run a lot better on a well aligned 64k stride raid5.
>
> In other words, partition tables, LVM, RAID, luks and filesystems know
> how to handle and profit from aligned larger chunks.
>
> In detail:
>
> As far as we can read mm/swapfile.c linux is only concerned about cpu
> page size and does not know anything about underlying
> chunk/sector/stride sizes and alignment.
>
> Therefore we think every small 1/2/4/8kiB page-sized write access leads
> to a read-modify-write cycle for the whole chunk, taking more then twice
> as long than simply writing the whole chunk at once.
>
> Questions:
>
> Is this the right place to ask?
>
> Does or could linux swapping make use of aligning chunks?
>
> And if, how?
>
> If not, would it be an improvement?
>
> Will this effect be mostly compensated by the block elevator?
>
> Does it make any sense to change the mkswap page size to the chunk size?
> We think those are two totally different beasts and should be left
> seperated.
>
> Is Linux already aware of chunk sizes within swap?
>
> How to set up and controlled by the administrator?
>

Hi Christian,

There has been a lot of work on alignment, Martin Petersen lead most of that and 
is probably the best one to ping.

Ric