[stgt] iostat shows all tgt I/O in 512 byte operations... how to coalesce?

Chris Worley worleys at gmail.com
Sat Sep 20 03:40:24 CEST 2008

I'm running CentOS 5.2 targets w/ a 2.6.24 kernel.  The initiator is
Win2003.  On the initiator side, the fs is formated NTFS w/ a 4K block
size (and the NTFS block size seems to have nothing to do w/ this

Watching iostat on the target side, everything is being written to the
underlying disk in 512 byte operations.

Best I can tell, it's the Linux side that's fragmenting the I/O.

I could get a lot better performance if these were coalesced into
larger, variable, block sizes (i.e. what's being written from the
initiator side is much larger blocks).

Is there something tgtd queries on the disk to get this information?
I don't see an fstat64 use of st_blksize in the source.

I can put a dummy md "linear" device atop the disk and set the MD
device's chunk size to 4K... then everything to the MD device (as well
as to the underlying disk) is passed in 4K blocks... which performs
much better (except even larger blocks would get better performance if
the user is writing larger blocks... and smaller blocks do a
read-modify-write that causes 3x the IO activity to perform).

Where is this getting fragmented, and any idea how to fix it?


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