I've been investigating an issue for a user who was seeing
his pool import hang after upgrading on FreeBSD. After
digging around it turned out the issue was due to lack
of free space on the pool.
As the pool imports it writes hence requiring space but the
pool has so little space this was failing. The IO being
a required IO it retries, but obviously fails again
resulting the the pool being suspened hence the hang.
With the pool suspended during import it still holds the
pool lock so all attempts to query the status also hang,
which is one problem as the user can't tell why the hang
has occured.
During the debugging I mounted the pool read only and
sent a copy to another empty pool, which resulted in ~1/2
capacity being recovered. This seemed odd but I dismissed
it at the time.
The machine was then left, with the pool not being accessed,
however I just recieved an alert from our monitoring for
a pool failure. On looking I now see the new pool I created
with 2 write errors and no free space. So just having the
pool mounted, with no access happening, has managed to use
the remain 2GB on the 4GB pool.
Has anyone seen this before or has any ideas what might
be going on?
zdb -m -m -m -m <pool> shows allocation to transactions e.g.
metaslab 100 offset c8000000 spacemap 1453 free 0
segments 0 maxsize 0 freepct 0%
In-memory histogram:
On-disk histogram: fragmentation 0
[ 0] ALLOC: txg 417, pass 2
[ 1] A range: 00c8000000-00c8001600 size: 001600
[ 2] ALLOC: txg 417, pass 3
[ 3] A range: 00c8001600-00c8003a00 size: 002400
[ 4] ALLOC: txg 418, pass 2
[ 5] A range: 00c8003a00-00c8005000 size: 001600
[ 6] ALLOC: txg 418, pass 3
[ 7] A range: 00c8005000-00c8006600 size: 001600
[ 8] ALLOC: txg 419, pass 2
[ 9] A range: 00c8006600-00c8007c00 size: 001600
[ 10] ALLOC: txg 419, pass 3
I tried destroying the pool and that hung, presumably due to
IO being suspended after the out of space errors.
Regards
Steve
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