Re: Putting very big and small files in one subvolume?

[Russell Coker <russell@coker.com.au>]

On Sun, 17 Aug 2014 12:31:42 Duncan wrote:
> OTOH, I tend to be rather more of an independent partition booster than 
> many.  The biggest reason for that is the too many eggs in one basket 
> problem.  Fully separate filesystems on separate partitions separate 
> those data "eggs" into separate baskets, so if the metaphorical bottom 
> drops out of one of those filesystem baskets, only the data eggs in that 
> filesystem basket are lost, while the eggs in the separate filesystem 
> baskets are still safe and sound, not affected at all. =:^)
> 
> The thing that troubles me about replacing a bunch of independent 
> partitions and filesystems with a bunch of subvolumes on a single btrfs 
> filesystem is thus just that, you've nicely divided that big basket into 
> little subvolume compartments, but it's still one big basket, and if the 
> bottom falls out, you potentially lose EVERYTHING in that filesystem 
> basket!

I'll write the counter-point to this.

If you have several partitions for /, /var/log, and /home then losing any one 
of them will result in a system that's mostly unusable.  So for continuous 
service there doesn't seem to be a benefit in having multiple partitions.

When you have to restore a backup in adverse circumstances the restore time is 
important.  For example if you have 10*4TB disks and need RAID-1 redundancy 
(which you need on any BTRFS filesystem of note as I don't think RAID-5 and 
RAID-6 are trustworthy) then an advantage of 5*4TB RAID-1 filesystems over a 
20TB RAID-10 is that restore time will be a lot smaller.  But this isn't an 
issue for typical BTRFS users who are working with much smaller amounts of 
data, at this time I have to recommend ZFS over BTRFS for most systems that 
manage 20TB of data.

If you have a RAID-1 array of the biggest disks available (which is probably 
the biggest storage for >99% of BTRFS users) then you are looking at a restore 
time of maybe 4TB at 160MB/s == something less than 7 hours.  For a home 
network 7 hours delay in getting things going after a major failure is quite 
OK.

Finally failures of filesystems on different partitions won't be independent.  
If one filesystem on a disk becomes unusable due to drive firmware issues or 
other serious problems then other filesystems on the same physical disk are 
likely to suffer the same fate.

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