[Linux-PowerEdge] Extremely poor performance with LVM vs. RAW disc

Linda A. Walsh dell at tlinx.org
Mon Nov 5 15:50:34 CST 2012

Dan Pritts wrote:
>> If you want to test disk speed -- you should pre-allocate the file as 
>> contiguous (you can
>> do this on xfs... dunnow about ext4)....then when you 'dd', use direct 
>> I/O -- and use
>> nocreat,notrunc options on 'dd', so you are reading/writing to the 
>> same area and aren't
>> exercising the file-system's ability to allocate space, nor the OS's 
>> buffer system.
> Or, just dd to and from a raw partition. 
> -- 
> Dan Pritts
> ICPSR Computing & Network Services
> University of Michigan
	No such thing anymore.

	There still is a a way to map a raw device to a block
device in the kernel using /dev/raw like you use /dev/loop.

	But by default, when you do block i/o to any normal block device,
block device, it goes through the linux block cache which slows things down
by 10-30% depending on your block device and the amount of data you are 

You certainly are NOT guaranteed strip-aligned reads/writes when using the
block cache.

dd if=/dev/sda of=/dev/sdb will go through block cache.

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