[pve-devel] [RFC storage] zfspoolplugin: check if mounted instead of imported

[Thomas Lamprecht]

On 18.02.21 12:33, Stoiko Ivanov wrote:
> This patch addresses an issue we recently saw on a production machine:
> * after booting a ZFS pool failed to get imported (due to an empty
>   /etc/zfs/zpool.cache)
> * pvestatd/guest-startall eventually tried to import the pool
> * the pool was imported, yet the datasets of the pool remained
>   not mounted
> 
> A bit of debugging showed that `zpool import <poolname>` is not
> atomic, in fact it does fork+exec `mount` with appropriate parameters.
> If an import ran longer than the hardcoded timeout of 15s, it could
> happen that the pool got imported, but the zpool command (and its
> forks) got terminated due to timing out.
> 
> reproducing this is straight-forward by setting (drastic) bw+iops
> limits on a guests' disk (which contains a zpool) - e.g.:
> `qm set 100 -scsi1 wd:vm-100-disk-1,iops_rd=10,iops_rd_max=20,\
> iops_wr=15,iops_wr_max=20,mbps_rd=10,mbps_rd_max=15,mbps_wr=10,mbps_wr_max=15`
> afterwards running `timeout 15 zpool import <poolname>` resulted in
> that situation in the guest on my machine
> 
> The patch changes the check in activate_storage for the ZFSPoolPlugin,
> to check if the configured 'pool' (which can also be a sub-dataset) is
> mounted (via `zfs get mounted <dataset>`).
> 
> * In case this errors out we try to import the pool (and run `zfs
>   mount -a` if the dataset is not mounted after importing)
> * In case it succeeds but reports the dataset as not mounted,
>   we run `zfs mount -a`
> 
> The choice of running `zfs mount -a` has potential for regression, in
> case someone manually umounts some dataset after booting the system
> (but does not set the canmount option)

Why not
# zfs mount <dataset>
?

> 
> Signed-off-by: Stoiko Ivanov <s.ivanov at proxmox.com>
> ---
> * sending as RFC since I have the feeling that I miss quite a few corner-cases.
> * an alternative to getting the mounted property via `zfs get` might be
>   parsing /proc/mounts (but zfs get mounted does mostly just that +stating
>   files in /sys, and 2 (potentially blocking) ioctls
> * could reproduce the issue with ZFS 2.0.1 and 0.8.6 - so my current guess
>   is the issue on the production box might be related to some other
>   performance regression in its disk-subsystem (maybe introduced with the
>   new kernel)
> * huge thanks to Dominik for many suggestions (including the idea of
>   limiting the disk-speed via our stack instead of dm-delay :)
> 
>  PVE/Storage/ZFSPoolPlugin.pm | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/PVE/Storage/ZFSPoolPlugin.pm b/PVE/Storage/ZFSPoolPlugin.pm
> index 105d802..32ad1c9 100644
> --- a/PVE/Storage/ZFSPoolPlugin.pm
> +++ b/PVE/Storage/ZFSPoolPlugin.pm
> @@ -525,8 +525,17 @@ sub activate_storage {
>      my ($class, $storeid, $scfg, $cache) = @_;
>  
>      # Note: $scfg->{pool} can include dataset <pool>/<dataset>
> -    my $pool = $scfg->{pool};
> -    $pool =~ s!/.*$!!;
> +    my $dataset = $scfg->{pool};
> +    my $pool = ($dataset =~ s!/.*$!!r);
> +
> +    my $dataset_mounted = sub {
> +	my $mounted = eval { $class->zfs_get_properties($scfg, 'mounted', "$dataset") };

quotes don't do anything here, just gets you a "undefined value in string ..." warning
if undefined (and underlying run_command does not change behavior either).

I.e.,

# perl -we 'use PVE::Tools qw(run_command); my $arg; run_command(["touch", "$arg"])'
Use of uninitialized value $arg in string at -e line 1.
touch: cannot touch '': No such file or directory
command 'touch ''' failed: exit code 1

vs.

# perl -we 'use PVE::Tools qw(run_command); my $arg; run_command(["touch", $arg])'
Use of uninitialized value $_[1] in exec at /usr/share/perl/5.28/IPC/Open3.pm line 178.
touch: cannot touch '': No such file or directory
command 'touch ''' failed: exit code 1

I like to avoid using quotes in such cases to avoid suggesting that they have
any use.

> +	if ($@) {
> +	    warn "$@\n";
> +	    return undef;
> +	}

I know this is just copied, but both should just use a simple
warn "$@\n" if ($@);

as the return is correctly using a defined check anyway


> +	return defined($mounted) && $mounted =~ m/^yes$/;
> +    };
>  
>      my $pool_imported = sub {
>  	my @param = ('-o', 'name', '-H', "$pool");
> @@ -538,13 +547,21 @@ sub activate_storage {
>  	return defined($res) && $res =~ m/$pool/;
>      };
>  
> -    if (!$pool_imported->()) {
> +    if (!defined($dataset_mounted->())) {

don't complicated boolean, drop the defined.

> +	# error from zfs get mounted - pool import necessary
>  	# import can only be done if not yet imported!
>  	my @param = ('-d', '/dev/disk/by-id/', '-o', 'cachefile=none', "$pool");
>  	eval { $class->zfs_request($scfg, undef, 'zpool_import', @param) };
>  	if (my $err = $@) {
>  	    # just could've raced with another import, so recheck if it is imported
> -	    die "could not activate storage '$storeid', $@\n" if !$pool_imported->();
> +	    die "could not activate storage '$storeid', $err\n" if !$pool_imported->();
> +	}
> +	if (!$dataset_mounted->()) {
> +	    eval { $class->zfs_request($scfg, undef, 'mount', '-a') };
> +	    if (my $err = $@) {
> +		# just could've raced with another import, so recheck if it is imported

but you do not (re-)check anything?

> +		die "could not activate storage '$storeid', $err\n";
> +	    }
>  	}
>      }
>      return 1;
>