Re: [PATCH][RFC]ext4 on 4k-sector drives without read-modify-write support

[Jeff Moyer <jmoyer@redhat.com>]

"Ted Ts'o" <tytso@mit.edu> writes:

> On Mon, Feb 28, 2011 at 04:10:44PM -0500, Jeff Moyer wrote:
>> > So it there a reliable way to detect that disk drives that don't have
>> > the Read-Modify-Write logic?  If that can be reflected up the I/O
>> > layer I can have mke2fs enforce that restriction, which currently we
>> > only enforce if the logical blocksize is 4k.
>> 
>> That's not enough, I'm afraid.  How do you deal with O_DIRECT, for
>> example?
>
> The drives do the read/modify/write, so O_DIRECT works fine from the
> OS's perspective.  There is some risk of torn pages if you get a crash
> at the wrong moment, but that's true whenver you have a misaligned
> partition.  (At least for the drives that do the RMW in the disk
> firmware.)

Sorry, I thought we were discussing devices that didn't do
read-modify-write:

> >> Unlike others drives with Advanced Format, it has not
> >> read-modify-write logic, so it can only operate with 4k
> >> blocks.

Also, mkp noted that the theory of operation was to issue aligned units
of 4k, but using 512 byte addressing.  So, unless you want to keep a
blacklist AND write a remapping layer, I'd say don't buy those drives.

Cheers,
Jeff