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

[Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>]

Dave Chinner put forth on 8/7/2010 5:13 AM:
> 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.

Thanks for the clarification/education Dave.

-- 
Stan