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What happens during MFSRestore?
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Posted by: KAMcGoo

This is just an academic question, because I have already done this:

I have completed 2 upgrades:

1. Added a WD1200JB as a B drive (following the MFSTools2 instructions) to my stock SD-DVR40

I then used my 141 hour for a week or so (and added about 95 GB of data)

2. Ran MFSbackup | MFSrestore from my Maxtor 40 + WD1200JB to a Pair of DiamondMax 9's

During restoration, I got the amount of data (approx 95 GB) being copied.

Did all of the 95 GB get copied onto A (Master), or does the Backup|Restore distribute the data betwen the drives?

Posted by: Robert S

The point you seem to be missing is that drives are divided into partitions and that while partitions can be created and destroyed quite easily, resizing them can be difficult or even impossible.

The first 9 partitions on the A drive hold the Linux and TiVo software. Those partitions have to be on the A drive. That's less than 1Gb of data and it's not data that depends on what the user does with his TiVo. All the user data is held in MFS partitions. MFS partitions exist in pairs. The first of each pair is called an 'Application Region' and the second a 'Media Region'. The App partition(s) hold the Guide DB and all the user data other than the recording streams themselves (note that the metadata required to turn the streams into viewable recordings are in the App partitions, which is why there can be show listed in Now Playing after you restore a compressed backup). Some TiVoes have only 1/2Gb of App partitions, but the vast majority have 1Gb, which may be a single partition or split over two partitions depending on the configuration.

When you upgrade your TiVo you add additional pairs of MFS partitions to expand the image. As the need for Application space does not depend on the space available for recordings, the new App partitions are tiny.

MFS Tools can not resize an MFS partition (neither can anything else). So, when you do a pipe transfer like this, MFS Tools copies the partitions from the source drive(s) on to the new one(s) and then adds a new pair or pairs of partitions to expand the image.

The TiVo partition tables are limited to 16 entries, which means that the A drive is limited to just three pairs of MFS partitions (9 system partitions plus 6 MFS). As current TiVoes ship with 2 pairs on their A drive, this can make it difficult to upgrade your TiVo more than once.

However, MFS Tools can copy partitions that were on a B drive to an A drive, or vice-versa. It is therefore possible to combine two drives into one. It is also possible to copy partitions off an A drive on to a B drive to free-up space in the partition table to enable further expansion of the A drive.

If you're really curious as to what happened, run mfsinfo on the drive sets - it should be possible to identify which partitions ended up where.

MFS Tools will try to fit all the partitions on to the A drive if it can, if it can't, then it will distribute the partitions over the two drives. If you use -x then it will pay attention to the need to create a new pair of partitions on the A drive, so if you do this it's important to copy with -x rather than copy and then expand with mfsadd.