[linux-lvm] LILO configuration for LVM "boot" filesystem
jackmc-linux-lvm at lorentz.com
Thu Jun 7 22:15:49 UTC 2001
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
3. It looks up the quotient in the LE to get a real device name and
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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