Re: How to track down abysmal performance ata - raid1 - crypto - vg/lv - xfs

[Dave Chinner <david@xxxxxxxxxxxxx>]

On Thu, Aug 05, 2010 at 03:33:17AM -0500, Stan Hoeppner wrote:
> Neil Brown put forth on 8/4/2010 5:24 PM:
> 
> > Both page-cache and read-ahead work at the filesystem level
> 
> Are you referring to /sys/block/sdx/queue/read_ahead_kb?  I'm pretty sure this
> works below the FS level and below the partition level.  This read_ahead works
> at the block device level.  At least for individual or JBOD.

That number is used to initialise the default readahead value for
any file descriptor opened on the filesystem. readahead is tracked
per-fd at the page cache level, so is effectively at the filesystem
level, not the block device.

> Are you saying this setting gets ignored by the kernel if/when mdadm, LVM,
> and/or crypto are used?

Only the value from the block device the filesystem sits on is used.
i.e. if you are using /dev/md0, then the filesystem uses the value
from /sys/block/md0/queue/read_ahead_kb and ignores all the ones set
on the /dev/sdX devices that make up /dev/md0.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx