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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