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Subject: Re: btrfs on whole disk (no partitions)
From: Chris Murphy <lists@colorremedies.com>
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Date: Sun, 22 Jun 2014 12:31:44 -0600
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On Jun 22, 2014, at 1:49 AM, Imran Geriskovan <imran.geriskovan@gmail.com> wrote:

>> The 64KB Btrfs bootloader pad is 8 sector aligned, so for 512e AF disks
>> there's no problem formatting the whole drive. The alignment problem
>> actually happens when partitioning it, using old partition tools that don't
>> align on 8 sector boundaries. There are some such tools still floating
>> around.
> 
> A 'somewhat' related question:
> 
> So called Advanced Format drives has 4K physical sector size,
> however they report 512B logical sector size.
> 
> How does linux kernel access those drives?
> 512B or 4K at a time?

Does anyone know if blktrace will intercept the actual SCSI commands sent to the drive? Or is there a better utility to use for this? When I use it unfiltered, I'm not seeing SCSI write commands at all.

This is the blktrace/blkparse for dd if=/dev/zero of=/dev/sda seek=430234064 count=8 oflag=direct
https://docs.google.com/file/d/0B_2Asp8DGjJ9c3J0UXpzUEhNaGs/edit?pli=1

This is the blktrace/blkparse for dd if=/dev/zero of=/dev/sda bs=4096 seek=53779258 count=1
https://docs.google.com/file/d/0B_2Asp8DGjJ9MnEwU0hDNE45dHc/edit?pli=1

The dd command clearly shows a difference, but when I look at the ATA command set I'd expect the Linux SCSI layer to turn these dd commands into the same ATA command: basically a write start LBA and sector count. And the LBA and sector count can only be in 512 byte sector addresses because that's all a 512e type of AF disk understands.

Chris Murphy