[mythtv-users] understanding storage group balancing

[Yeechang Lee]

Simon Hobson says:
> As I read the link above, specifically the advanced notes, it would
> appear that Balanced Disk I/O is now equivalent to Combined if
> either all storage is local or all is remote.

Author of the balanced disk I/O scheduler here. The answer to your question is "No". That you thought "Yes" is quite understandable, though, because I've been where you are.

The short version is that combination did not (and does not, unless it has since been fixed) work the way the wiki and documentation describes it, causing big problems for anyone with multiple storage directories in both local and remote locations. If you want the whole sordid story, read <https://lists.archive.carbon60.com/mythtv/users/346903/?page=1;mh=-1;> from back when combination was the only algorithm (and thus not called "combination").

Balanced free space and balanced percent free space were created after the above thread to fix the blatantly broken behavior of combination, with balanced free space being made default as it is the one that makes the most intuitive sense for most people. I submitted balanced disk I/O later, after experiencing problems with balanced free space when multiple tuners started writing to the same RAID 6 filesystem at the same time.

> If so, then I guess for many (most ?) of us, Balanced Disk I/O makes
> the most sense

One can make the case for either it or balanced free space as the best default choice. Balanced free space is the best fit for the typical user with one or two tuners who occasionally adds a new drive, and expects that one to be used for all new recordings until it is as full as the older filesystems. This also eliminates expiration of older programs on older filesystems to make room before free space parity occurs. Balanced disk I/O does not behave so intuitively, but is preferable for those using more than (say) four or five tuners and/or RAID 5/6 (inherently slower than single drives). In practice with balanced disk I/O, any new drive will get used more often than older filesystems and thus eventually also reach parity in terms of free space, albeit more slowly than with balanced free space.

-- 
Frontend:		Apple MacBook Pro 2012, Nvidia Shield 2017
Backend:		HP Microserver N40L 1.5GHz with 4x3TB HDDs
Tuners:			Two over-the-air ATSC inputs with multirec