On 12/14/2013 04:28 PM, Hans-Kristian Bakke wrote:
I would normally expect that there is no difference in 1TB free space
on a FS that is 2TB in total, and 1TB free space on a filesystem that
is 30TB in total, other than my sense of urge and that you would
probably expect data growth to be more rapid on the 30TB FS as there
is obviously a need to store a lot of stuff.
Is "free space needed" really a different concept dependning on the
size of your FS?
I would suggest there just might be a very significant difference. In
the case of a 30TB array as opposed to a 3TB array, you are dealing with
a much higher ratio of used space to free space. I believe this creates
a higher likelihood that the free space is occurring as a larger number
of very small pieces of drive space as opposed to a 3TB drive where
1/3rd of the drive space free would imply actual USABLE space on the
drives. My concern would be that with only 1/30th of the space on the
drives left free, that remaining space likely involves a lot of very
small segments that create a situation where the filesystem is
struggling to compute how to lay out new files. And, on top of that,
defragmentation could become a nightmare of complexity as well, since
the filesystem first has to clear contiguous space to somewhere in order
to defragment each file. And then throw in the striping and mirroring
requirements. I know those algorithms are likely pretty sophisticated,
but something tells me that the higher the RATIO of used space to free
space, the more difficult things might get for the filesystem. Just
about everybody here knows a whole lot more about this than I do, but
something really concerns me about this ratio issue. Ideally of course
it probably should work, but its just got to be significantly more
complex than a 3TB situation. These are just my thoughts as a
comparative novice when it comes to btrfs or filesystems in general.
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