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[linux-lvm] Strange behaviour with initramfs and read-ahead settings



Hi list,

I am currently running the 2.6.32-rc8 kernel with lvm2 version 2.02.54-1
on debian unstable.
Since my root is on LVM as well I use an initramfs for booting. The
setup I have consists of 5 disks in a RAID-5 with one PV on top of it.
Now for my problem. The md device correctly sets the read-ahead to 4096.
If I understand correctly lvm should get this value and set the
read-ahead on the LVs accordingly. This is not working correctly for
all my LVs.
Apart from the FIRST LV on the disk all the others show

Read ahead sectors     auto
  - currently set to     256

Since this did not seem to work I changed the RA with lvchange -r 4096
and rebooted. Swap now showed 

Read ahead sectors     4096

while the other LVs still showed


Read ahead sectors     4096
  - currently set to     256

lvs -o lv_read_ahead after boot 

 Rahead 
 2.00m 
 2.00m 
 2.00m 
 2.00m 
 2.00m

blockdev --getra  showed 256 for each LV apart from the first one.

For testing purposes I took one LV (home) offline and back online again

lvchange -an;lvchange -ay

After that the readahead was set correctly to 4096. I am not sure what's
happening here. I would understand a problem with getting the values
automatically but right now they are saved in the metadata but are still
set to 256. 

I am glad for any help on this topic, in the mean-time I am using
blockdev --setra to work around this problem.

Kind regards,
Michael Guntsche


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