why iscsi lun are taken offline when full?

[fcantisano]

Hi to all,

i'd like to know the technical reasons (if any) that brings an iscsi offline when it gets full.

It's possible to configure the netapp to keep a volume always online (also when it's full) ?

Thanks

[ashleycook]

This is a mechanism to prevent writes being lost isn't it? By taking the lun offline, it prevents any further writes from the host to the storage.

Say you had a 100GB volume, containing a 75GB lun. In a configuration where there is no fractional reserve or snapshot reserve configured, this allows 25GB of snapshots. Once you consume this 25GB, no further writes are possible, due to writes always being to new blocks. Consequently, the lun is taken offline to prevent any writes being lost and to maintain data integrity.

Probably the options to be considering if you want to avoid the lun being taken offline are snapshot autodelete and volume autosize, those will buy you some time to react to a volume filling up.

[fcantisano]

Hi,

I know this, but in some cases (and with no snapshot configured at all)  one could prefer to have application that fails writes (stopping themselves) and other applications, on same lun, continue to perform reads letting to the users to remove/purge olde data to obtain free space.

Regards.

[ashleycook]

I think perhaps if you could describe your configuration it may help clear things up. If you have a 100GB lun in a volume which is 100.1GB (or maybe even 100GB) with the fractional reserve set to zero, snapshots disabled and the snapshot reserve set to zero, I would not expect that lun to be taken offline when it reaches 100% file system utilization.

To achieve what you are describing sounds like it would require OS/application level monitoring of the file system utilization. It is not out of the question, but I don't see how the storage array would be able to control this.

Your OS/application would need to sit between your storage and users, and monitor the space utilization of the file system. When space becomes low it could reject further writes, allowing users to perform deletions.

Personally, I think that process is beyond the scope of the storage array to manage.

[ivisinfo]

I think the correct question woulc be "why iscsi lun are taken offline when volume gets full?".

I don't think that a LUN gets offline if it gets full and there is still space in the volume. Assuming there are no snapshots, maybe you should give a few extra space to the volume, so that a full LUN does not trigger a full Volume. With snapshots enabled you should calculate better the space so that you do not end up with a full volume.