Re: very low performance on SCSI disks if device node is in tmpfs

[Andrew Morton]

Olaf Hering <olh@xxxxxxx> wrote:
>
> Any ideas why the location of the device node makes such a big
> difference? I always wondered why my firewire is so dog slow with 2.6.
> Now I know the reason: /dev is in tmpfs.
> I dont see that with IDE disks, only with SCSI.

This is truly bizarre.  Reading /dev/sda I get 24MB/sec at 700 context
switches/sec.  Reading /mnt/tmpfs/sda it's 14MB/sec, 7000 switches/sec. 
/mnt/ramfs/sda is slow too. /mnt/hda5/sda is fast.


I'd assumed that the kernel got the backing_dev_info's screwed up and the
tmpfs node isn't doing readahead but that appears to not be the case
(/dev/sda is still fast with zero readahead).


You really, really get the weird-bug-of-the-month award for this one.  I'll
poke at it some more later on.
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