[Lustre-discuss] Unusual Block Allocations on OSTs

[Roger Spellman]

Hi,

 

I see some unusual block allocations on my OSTs, and I was wondering if
someone could explain to my why, and help me to fix a performance
problem.

 

In order to track down a performance issue, I ran the following test:

- I reformatted my OSTS (I have 4 OSTs).

- I created a 10G file on each OST.

- I ran dumpe2fs to see if I had some unusual fragmentation going on.

 

dumpe2fs shows the free blocks per group.  There are 25900 groups on
each OST (each OST is about 3.5 T big).

 

On my fastest OSTs, the allocated blocks are from groups 6864 - 6950.  I
would have thought that blocks would be allocated from group 0.

 

On the slowest OSTs, the block are allocated from groups 24053 - 24138,
which is near the end of my RAID Array (and presumably, near the end of
my disks!)

 

The other OSTs allocate from other groups.

 

 

This leads me to several questions:

 

1.  Why does each OST allocate from different locations?

2.  What subroutine in the code controls this?

3.  Is there a way to control this, to improve my performance?

 

Thanks.

 

-Roger

 

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