Re: [PATCH V2] Btrfs: Full direct I/O and AIO read implementation.

[Christoph Hellwig <hch@infradead.org>]

On Mon, Feb 15, 2010 at 05:26:34PM -0500, jim owens wrote:
> My understanding is the current 4k drives normally operate in
> 512 byte read/write access mode unless you set them to run
> as 4k only.
>
> In 512 byte mode, they buffer internally on writes.  It is probably
> just as safe as any other drive on a power hit, as in anything may
> be trash.
>
> btrfs read of 512 byte boundaries is safe because we only write
> in 4k boundaries (hopefully we can detect and align on the drive).

There are drives that still have 512 byte logical, but 4k physical
blocks, this includes all the consumer (SATA) drives.  You can also
have drives with 4k physical and logical block size, this includes
many S/390 DASD devices, and also samples of enterprise SAS drives.

The logical block size is the addressing limit for the OS, so your
above scenario is correct for the 512 bye logical / 4k physical
devices, but not the 4k logical / 4k physical devices.  Nevermind
other corner cases like 2k block size CD-ROM which could in theory
be used in a read-only btrfs filesystem (very unlikely, but..).

So no, you really can't go under the bdev_logical_block_size()
advertized by the device, and that may as well be over 512 bytes.