Juergen Nickelsen wrote:
Is there any limit on the number of snapshots in a file system?
The documentation -- manual page, admin guide, troubleshooting guide
-- does not mention any. That seems to confirm my assumption that is
is probably not a fixed limit, but there may still be a practical
one, just like there is no limit on the number of file systems in a
pool, but nobody would find having a million file systems practical.
I have tried to create a number of snapshots in a file system for a
few hours. An otherwise unloaded X4250 with a nearly empty RAID-Z2
pool of six builtin disks (146 GB, 10K rpm) managed to create a few
snapshots per second in an empty file system.
It had not visibly slowed down when it reached 36051 snapshots after
hours and I stopped it; to my surprise destroying the file system
(with all these snapshots in it) took about as long. With ``iostat
-xn 1'' I could see that the disk usage was still low, at about 13%
IIRC.
So 36000 snapshots in an empty file system is not a problem. Is it
different with a file system that is, say, to 70% full? Or on a
bigger pool? Or with a significantly larger number of snapshots,
say, a million? I am asking for real experience here, not for the
theory.
The most I ever had was about 240000 on a 2TB pool (~1000 filesystems, x
60 days x 4 per day). There wasn't any noticeable performance impact,
except when I built a tree of snapshots (via libzfs) to work out which
ones had to be replicated.
Deleting 50 days worth of them took a very long time!
--
Ian.
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