From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4BA3EC26.7000302@cfl.rr.com> Date: Fri, 19 Mar 2010 17:27:02 -0400 From: Phillip Susi MIME-Version: 1.0 References: <4BA3BEBE.2010204@cfl.rr.com> <20100319192537.GA7345@redhat.com> In-Reply-To: <20100319192537.GA7345@redhat.com> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Data alignment Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" To: LVM general discussion and development Cc: Mike Snitzer On 3/19/2010 3:25 PM, Mike Snitzer wrote: > Any reason why you'd like to use DOS partitioning (first primary > partition starting at 63rd sector)? Convention. I am looking into using GPT instead though. > Anyway, if you have a recent kernel (e.g. 2.6.33) you'll be in good > shape regardless. If you create a partition on the device (using DOS > partitions) the kernel _should_ be aware of where the partition starts > and tell you how far you'd have to shift the start of your LVM2 PV data > area to get it perfectly aligned relative to the underlying physical > block size. Check for example: > # cat /sys/block/sda/sda1/alignment_offset > > But if your device is using 512b physical_sector_size you'll just have a > 0 for alignment_offset. Check physical_block_size with: > # cat /sys/block/sda/queue/physical_block_size 0 alignment, physical block size 512. > Also verify that your SSD device is naturally aligned (aka > alignment_offset=0); I'd wager it is naturally aligned: > # cat /sys/block/sda/alignment_offset Kernel thinks so, and based on performance tests it appears so. > All said, even if you have an older kernel, to manually get what you > want (shift start to account for DOS partition at 63rd sector, align PV > pe_start on a 512K boundary), please try: > # pvcreate --dataalignmentoffset 512b --dataalignment 512K ... Won't that just add one sector to the start, placing it at sector 1025? How does an alignment offset of 1 sector account for the partition starting on sector 63?