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Re: [linux-lvm] LILO configuration for LVM "boot" filesystem



Big Brother tells me that Steve Wray wrote:
> > 
> > LV are generally slower than other types of storage.  Since the
> > boot partition is generally cyl 0 it's on the fastest storage
> > available (short of tmpfs).  
> 
> Proportionally, how much slower is it? 50%? 25%? 12%? 
> less than 1% slower?

    Not even.  People don't realize how trivial LVM access actually are.
Suppose the OS has decided it needs to access block n of device foo. It
calls the driver to grab it.  If foo is sda1, then a read request is
scheduled (unless it is in the cache).  If foo is rootvg/usr_lv, then

1. It gets a pointer to the LE table (which it already has from the
device data when the read request is called in the lvm driver).
2. It executes a division, keeping the remainder (n divided by the
  LE size).
3. It looks up the quotient in the LE to get a real device name and
   offset.
4. It adds the remainder to the offset and calls the real low-level driver.

    In other words, every block read that goes through LVM adds basically
a division, a memory lookup and an addition.  The time it takes to transfer
the block from the drive is EONs compared to the time it takes to perform
a couple of arithmetical computations.

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