Question: I have 100GB LUN inside a 200GB volume - I am trying to expand it using SnapDrive but I can only increase it by a few GB. Can't I expand it by more?
Dr. Toaster's Answer: It is important to first understand the concept of disk space reservation. The simplest way to explain that is to think of a regular magnetic disk drive - it has addresses that hosts can refer to in order to read or write data - and hosts can always send I/O write commands to these data block addresses again and again - that's the whole idea of an address space.
The WAFL filesystem has a different write allocation policy - when you keep snapshots - point-in-time views of the same filesystem, WAFL keeps blocks untouched, so that one can recover from snapshots that reference these blocks.
So to connect the story here - LUNs are implemented on top of WAFL - so when snapshots are being taken, if a host (that is totally unaware of this virtualization) is writing data consistently into a LUN, more and more data blocks will be held "captive" by the snapshots. This is where space reservation algorithms kick in - to protect this behaviour from causing SCSI write errors to LUNs - by default, every LUN consumes its original size
plus another 100% of its size as well as a protection against this rare case of frequent writes into a LUN on a volume with many snapshots created.
The simple solution to enable the expansion of the LUN is to enlarge the size of the underlying volume - with FlexVols that would be an easy change. If there is not enough disk space in the aggregate, one can reduce the amount of space reservations per volume using a command such as:
vol options vol_name fractional_reserve 80
where
80 is a number below 100. Note that if snapshots will be taken and the rate of changes in the volume will be higher than 80% then SCSI writes to the LUNs may fail - in which case the filer will take the LUNs in the volume offline, and manual action will have to take place to clean up space (most likely by deleting some snapshots) and online the LUNs again (
lun online lun_pathname).
A few other important notes:
1. Use the following commands to review the status of space reservation:
df
df -r
snap delta
2. Data ONTAP 7.2 adds another solution which is to allow volumes to automatically clean up old snapshots and/or grow:
vol options vol_name volume_grow on
vol autosize vol_name -m 1000g -i 1g on
where
1000g is the maximum size that the autosize feature will allow the volume to grow to, and
1g is the increment.
The autosize feature will try to increment the volume size by 1GB increments, and if the aggregate is full it will try to delete snapshots. It is also possible to start with deleting snapshots by using the
snap_delete policy instead of the
volume_grow I suggest above.
