On 2017-02-16 15:13, E V wrote:
It would be nice if there was an easy way to tell btrfs to allocate
another metadata chunk. For example, the below fs is full due to
exhausted metadata:
Device size: 1013.28GiB
Device allocated: 1013.28GiB
Device unallocated: 2.00MiB
Device missing: 0.00B
Used: 981.94GiB
Free (estimated): 15.16GiB (min: 15.16GiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 510.31MiB (used: 0.00B)
Data Metadata System
Id Path RAID1 RAID1 RAID1 Unallocated
-- --------- --------- --------- -------- -----------
1 /dev/sdv1 505.63GiB 1.00GiB 8.00MiB 1.00MiB
2 /dev/sdw1 505.63GiB 1.00GiB 8.00MiB 1.00MiB
-- --------- --------- --------- -------- -----------
Total 505.63GiB 1.00GiB 8.00MiB 2.00MiB
Used 490.47GiB 510.88MiB 96.00KiB
I can delete a multi GB file and get several GB of unallocated space,
however if I try and copy big files to it again the same exact thing
happens. However, if I play with balance and deleting files and such
and manage to get it to allocate another metadata chunk while there is
unallocated space then the filesystem will happily fill up all of the
data chunks. Failing an automatic allocation out of global reserve, or
saving metadata as soon as unallocated space is available it would be
nice if I could just delete a file and then tell btrfs to allocate
more metadata immediately. Makes sense? No idea how easy this would be
to do, but seems like it should be a simple thing btrfs file could do.
The potentially tricky bit about this is that BTRFS (at least, with
recent kernels) will deallocate completely empty chunks. The horribly
ironic part of this is that that behavior got added to help avoid
situations like this. There is technically logic to allocate extra
metadata chunks for every few data chunks that get allocated (and I
think the reverse too), but because of the auto-deallocation behavior,
this ends up just spinning and wasting cycles (and bandwidth). I would
personally love to see the following happen in regard to this all:
1. Add a switch to disable the auto-deallocation (ideally a mount
option). This bit _should_ be pretty easy, but I'm not certain.
2. Disable the auto-allocation of chunks of type X every N chunks of
type Y that get allocated when the auto-deallocation is enabled. This
one should in theory be pretty easy too.
3. Add a tool (not sure where exactly makes the most sense) to force
allocation of a specific chunk type. This would also be insanely useful
for debugging, but is probably the hardest part (would likely need a new
ioctl).
4. Possibly add an option to reserve some percentage of the space to
only be used for System and Metadata chunks. This would help prevent
this kind of thing from happening. I'm not entirely sold on this right
now, but it _seems_ like a generically good idea.
Sadly, I don't currently have the time to work on any of that myself,
and there are other things that are higher priorities, so I don't know
when (if ever) this may actually happen. I might have some time to look
into the first two things I listed some time soon, but I don't know how
soon that might be.
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