[mythtv-users] SMR drives on Mythtv

[Simon Hobson]

Michael T. Dean <mtdean at thirdcontact.com> wrote:

> Does that mean that SMR uses SMR-specific recording heads, and any other recording method specified is not shingled?

The key thing about shingled drives is that they can make use of the physics that makes it possible to have a read head that works with a narrower track than the write head. So you write the narrowest track that you can make a head for, but instead of leaving a guard zone and then writing another track - you write the next track so it overlaps the first one, leaving a narrower track which is all that's needed for the read head.

The problem is that you can't re-write anything but the last track in the group* - so you have to re-write the whole track group if you change any part of it. This typically means reading the whole track group, updating (in the buffer) the sectors you've changed, and then writing it out again - normally to different track group on the disk (so you still have the original if the power fails mid operation).

It is this "read a load of data; write it out again" that makes these drives stop processing requests for seconds at a time - they are having to move a chunk of data about internally.

* Obviously it would be impractical if you had to re-write the whole disk, so there are multiple zones with guard zones between them.